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Old 21st Apr 2017, 2:21 pm   #1
mhennessy
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Default Radio sound quality - some measurements

Prompted by a recent mention of loudspeaker types as used by Hacker in the Sovereign II, and in particular a comment that the Sovereign III might sound better. I've expressed my subjective thoughts about radio sound quality plenty of times, but I've been meaning to do some basic objective measurement to support these views for a while now...

This was just a quick 15 minutes in-between jobs, so I won't pretend that these measurements are in any way authoritative or rigorous. However, they do agree with my subjective impressions, and illustrate some interesting differences between sets. The sets tested were just the first few I could easily lay my hands on without venturing up into the attic or routing through boxes, etc. I can add to this in the future though.

The first measurement was the loudspeaker+cabinet only (power amp straight into the loudspeaker terminals). Then I did an "end to end" test - this involves turning the test stimulus into FM, then receiving that signal on the FM tuner. To provide FM pre-emphasis, I used my Radiometer SMG40 prior to my FM generator. This result was close enough to the "raw" response that I decided to omit the speaker-only tests subsequently.

The measurement was made using Omnimic, which consists of a calibrated microphone, software, and a CD containing the test stimulus. This is something I've used for many years, and have good levels of confidence in it. I chose to use a quasi-nearfield measurement at 30cm to get a reasonable compromise - usually you'd measure at 1m, but the room reflections distort the results too much unless you use gating, which removes the LF information. The grilles prevented a true nearfield measurement, and the slots, etc, behind the grilles would also cause problems.

I'll start with the RP72.

My objection with these sets has always been the upper midrange - there is a "honk" which can't be removed with the treble control. It doesn't seem to bother plenty of folk though, so perhaps it's just me?

The black trace is the raw loudspeaker. The red trace is the end-to-end result, with the tone controls in their central position. The blue trace is after I've adjusted the tone controls to get the best subjective result.

The first surprise is just how quickly the HF drops after 5kHz. Listening "raw", it sounds like an AM radio! The fall at LF - below 400Hz - is to be expected (Google "Baffle step correction").

The red trace shows some lift centred at 4kHz. I haven't measured the electrical frequency response at the loudspeaker terminals, but would expect that to be apparent there. Otherwise, things agree closely.

After adjustment, the best I could get was the blue trace. I've turned up the treble to try to get some HF, but that lifts the 4kHz peak by large amount - as you'd expect. At 10kHz, things are 3dB down compared to 1kHz, and the 4kHz peak is about 10dB up! Note also that the LF is still pretty poor after adjustment, although there is at least some energy below 100Hz.

OK, this isn't meant to be a hi-fi speaker, so perhaps we have unrealistic expectations? Well, let's look at some more...
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Old 21st Apr 2017, 2:38 pm   #2
mhennessy
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Default Re: Radio sound quality - some measurements

The RP25.

This is an example with the Goodmans loudspeaker. There are 2 versions of this fitted to the RP25 - early ones have 3 bolts on the rear of the magnet, later ones are without bolts. This is the latter.

Frankly, the Goodmans is a great AM speaker, but not so good for FM sets. Hacker recognised this, and replaced it with a Celestion model shortly before they changed to the RP25A. I don't have an example of this that is easily accessible, so I'll have to come back to that...

Subjectively, the Goodmans lacks treble. The bass is a bit tight too - earlier "bolted" models are more generous in that area. So what do the measurements tell us?

As mentioned earlier, I skipped the "raw", direct-to-loudspeaker measurements, and took two - tone controls set to flat (which is minimum on many Hacker sets, though not the RP72) - black trace - and after the tone controls had been set for best results on music (blue trace). And from here, I decided to use 1kHz as a reference point for both curves.

As expected, we see a great big lump in the middle (400Hz to 5kHz), with the bass and treble rapidly rolling away either side. But this one does go up reasonably far - better than I expected, and better than the RP72 unit, and it might be possible to use some dedicated equalisation to bring it up. It would be easier to add a small tweeter, perhaps.

At the bass end, we see the sharp rolloff below resonance (~100Hz).

Having adjusted the tone controls, things improve at the low end, and the HF is extended to just over 10kHz, but the 9dB spike at 3-5kHz is there, and it's not pretty.

So, slightly better than expected, but not a great result. Hacker can do better
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Old 21st Apr 2017, 2:47 pm   #3
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Default Re: Radio sound quality - some measurements

Next, the RP37A.

This VHF Herald is fitted with the large-magnet Elac - the best of them. Some sets were fitted with the same Celestion that the later Sovereign II sets got. The original VHF Herald came with the Goodmans, which we now know is best avoided...

As before, black is the "flat" position (bass/treb at min), and blue is the measurement after tweaking for best subject results.

This is more like it. At the bass end, look at how extended it is! Really quite incredible for a portable radio. And at the same time, plenty of treble extension - it reaches out to beyond 12kHz before dropping away significantly. There are a couple of dips (at 1.5kHz and 7kHz), but dips are far more preferable to peaks. And it's entirely possible that these dips are caused by the cabinet and/or grille.

With the tone controls adjusted, it's flatter between 75Hz and 12kHz than a lot of "hi-fi" speakers that I've measured. A fine result. This is an easy set to live with - and has long been one of my personal favourites.
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Old 21st Apr 2017, 2:54 pm   #4
mhennessy
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Default Re: Radio sound quality - some measurements

The RP38A.

This set has a strong reputation for sound quality, and deservedly so. The Elac speaker does a fine job, and I've always enjoyed it.

Looking at the curves, the overall trend is similar to the RP37A, but with less bass extension (not surprising), but still a decent top end. There are dips in similar places, which makes me curious to re-test these units in isolation next time I have cause to remove them from the cabinets.

Basically, +/-4dB from 75Hz to 12kHz. Really very good indeed. Especially given that the Hunter was the "economy" set, so the Elac was presumably cheaper than the Celestion fitted to the more upmarket models...
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Old 21st Apr 2017, 3:08 pm   #5
mhennessy
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Default Re: Radio sound quality - some measurements

The Roberts R707.

This is another set with a strong reputation for sound quality. And it does extremely well in these tests, happily. But with one important caveat: be careful with the tone controls! You can make this set sound horrible. There is too much range with both, and the amplifier/speaker combination doesn't cope well. This set - like the RP72 - uses Baxandall tone controls, but really needs padding resistors at the ends of the pots to restrict the range to something sensible...

This measurement was complicated by the loudness contour on the volume control. I set the volume to the mechanic centre, so we're seeing the effect of that on the measurements. The tone controls don't have a pointer, so to put those in their mechanical centre, I used stickers as temporary pointers.

The black trace - helped by the loudness tap - is really not bad. A very gentle tweak of the bass and treble controls ends up with a surprisingly flat response. These ended up being somewhere between 2 and 3 o'clock on their rotation - as mentioned, you have to be careful with these. Now, these sets roll away at HF a bit early, and the bass is a bit "bumpy" (high Q driver), but ignoring the harmless 1.6kHz dip, this set can be flat +/-4dB from between 70Hz and 11kHz.

Unfortunately, things change when the volume control is advanced past the mechanical centre, as the loudness tap reduces its influence on the response. For brevity, I didn't investigate this today, but could do in the future.

So, a good result!
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Old 21st Apr 2017, 3:35 pm   #6
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Default Re: Radio sound quality - some measurements

Panasonic RF-1105 and RF-1630.

For context, a couple of "lesser" sets. These are very basic models with tiny round speakers. But I actually really like these sets. Why? Incredibly FM sensitivity. That's really important to me, given that this is such a poor area around here. For use in the bathroom, or when doing DIY, these are what I pick.

The RF-1105 has a tone control, and no way of knowing which is the "flat" setting. So I simply adjusted it by ear. The RF-1630 has no tone control.

Obviously they are both limited in the bass. Taking the RF-1105 first, after 150Hz it's within +/-3dB all the way to 10kHz. The dip at 2kHz is harmless, but the peak at 6kHz is less welcome. Turning up the tone control brings up the 10KHz+ region quite nicely, but he 6kHz peak comes up with it. A simple notch filter would tame that nicely.

The RF-1630 is even smoother, but severely lacking in bass. It reaches 13.5kHz before falling away. Lovely sets for what they are...
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Old 21st Apr 2017, 3:43 pm   #7
mhennessy
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Default Re: Radio sound quality - some measurements

The Roberts R505.

Another basic set. Again, just a tone conrol, so adjusted by ear before measurement. This set is quite small, so lacks bass extension, and also HF. Within the 300Hz to 8kHz region it's smooth enough, so fine for Radio 4, but not a set for rock music!
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Old 21st Apr 2017, 3:56 pm   #8
mhennessy
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Default Re: Radio sound quality - some measurements

The Roberts R727.

How about something more modern? Surely this will be better, right?

Hmm - proof that the '80s wasn't a great decade, perhaps. This one was quite a surprise. Admittedly, I'd not bought this with great expectations - it was only a tenner at the 2011 NVCF, but it is in great condition. It's only out of the attic because of a thread on here a while back.

So there's clearly something wrong with the bass tuning. The giant dip between 1-2kHz is better than a peak, but a bit wide really. Things are better either side of that, but the sharp treble roll-off surprised me, given that everyone wanted treble in the '80s. Overall, this set sounds like something half its size.
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Old 21st Apr 2017, 4:20 pm   #9
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Default Re: Radio sound quality - some measurements

The Gundig Yacht Boy N210.

A set I've had plenty to say about in the past. This uses an astonishing amount of equalisation - more than the plastic cabinet can bear with today's compressed broadcasts. Actually, it sounded better today that I've ever heard it sound before, as I'm playing straight from a CD player with no processing.

The problem is the loudness taps (there are 2). The amount of bass boost provided by these is just astonishing (I've posted simulations elsewhere on the forum), and while these were perhaps OK at the time, today the amplifier clips before we get past the taps.

I've taken 2 measurements - the red one shows the full effect of both taps, and the black line is at half volume, where things are commendably flat. There is still a large amount of electrical boost there, but that's compensating for the falling response of the loudspeaker, naturally. Between the red and black lines, there's about 10dB at 100Hz. It really is astonishing, and I can't imagine how anyone thought that would be a good idea.

If you can advance the volume control enough (reducing the gain of the power amp might be a good plan), then things are quite reasonable. The HF rolls away quite sharply above 10kHz, and while the tone control can bring up the level of 10kHz, it also brings up that peak at 6.5kHz. The 1.5kHz dip is pretty benign...

That's probably enough for now, but thoughts, comments and requests are welcome. Hope it's been of interest

Mark



EDIT: Ignore the sub-50Hz stuff on the red trace - that's traffic noise (the gain was turned up because it was a low level measurement)
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Old 21st Apr 2017, 5:56 pm   #10
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Default Re: Radio sound quality - some measurements

Very interesting, you are clearly a very industrious fellow Mark.

Assuming the Elac is the best speaker does that mean that transplanting an Elac into an RP25 (for example) would have the desired effect? I assume the sound quality is down to amp/speaker combination.

Forgive me for suggesting such vandalism ......
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Old 21st Apr 2017, 6:02 pm   #11
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Default Re: Radio sound quality - some measurements

Quote:
I assume the sound quality is down to amp/speaker combination.
More likely the 'speaker box combination (unless you have a really bad amplifier).
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Old 21st Apr 2017, 6:17 pm   #12
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Default Re: Radio sound quality - some measurements

Quote:
Originally Posted by merlinmaxwell View Post
Quote:
I assume the sound quality is down to amp/speaker combination.
More likely the 'speaker box combination (unless you have a really bad amplifier).
I see your point but I was thinking Hacker specifically and quite a few of the boxes of that period were very similar materials and construction.
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Old 21st Apr 2017, 6:31 pm   #13
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Default Re: Radio sound quality - some measurements

Quote:
Originally Posted by nebogipfel View Post
Assuming the Elac is the best speaker does that mean that transplanting an Elac into an RP25 (for example) would have the desired effect?
I seem to remember a problem there would be that the RP25 chassis doesn't leave room for that Elac magnet.

Thanks, Mark: fascinating stuff, reinforcing my resolve that one of these days I must track down an Elac-equipped RP37A - somehow it's always been Celestion ones that have found their way here.

I've wondered over the years whether something was amiss with the only Yacht Boy 210 I've ever owned, after which I couldn't face another, but on the evidence here perhaps it was perfectly fine. Nothing against Grundig, the older Satellits and Ocean Boys seem very mannerly and are a joy to listen to and use.

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Old 21st Apr 2017, 6:53 pm   #14
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Default Re: Radio sound quality - some measurements

Excellent work Mr. H.

I've always very much favoured the RP72 (I'm listening to 'Tone' on R2 on one as I type) and also the R707, and have often wondered if this is due to the influence of the late great Peter Baxandall.

Maybe it is the 'sound of the sound' that I like.
http://www.douglas-self.com/ampins/pseudo/subjectv.htm

I agree on your comments on Goodmans (with or without bolts) being somewhat 'grunty' and lacklustre.

And yes, the VHF Herald is a bit like the 'Cooper S' of the Hacker world.

Never overwhelmed with any of the Grundigs, although the Concert Boy probably the best of them.
And as to the build quality......

Not overly taken by any of the Hunters; I currenly only own one (an earlyish one) but have owned most of the different varieties over the years.

For me it's been divide and conquer; I've gone from a collection of 500 of everything, to just a few I really like.

Oh, and no; I can't see that an Elac from a VHF Herlad will fit into a Sovereign, without major surgery.
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Old 21st Apr 2017, 7:37 pm   #15
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Default Re: Radio sound quality - some measurements

For completeness, we need to include measurements of the Celestion. So, after much hunting through boxes, I eventually found one:

The Open University RP37A (with Celestion).

This one isn't terrible by any means, but has a surprising lack of treble extension. As before, the traces are no EQ (bass/treble at min), and after adjustment by ear for best sound quality. I wanted more treble, but couldn't because of the painful 4-5kHz peak...

Out of interest, I put the Goodmans (blue), Celestion (black), large-magnet Elac (green) and the small-magnet Elac (red) on the same graph. Sorry it's a bit of a mess!

At low frequencies, they are pretty similar, with the large-magnet Elac giving a bit more extension, and the Hunter Elac having a bit more output (higher Q) at 100Hz. You'll note similarities between all 4 in this region - such as at 300Hz, 400Hz and 600Hz - which will be in-part caused by the sizes of the boxes (all much the same).

It's really above 1kHz where they vary most, and this is where we're most critical*. The Celestion and Goodmans have the largest peaks in the 3-5kHz region before rapidly falling away at 9kHz. The large-magnet Elac shows a much smaller peak at 4.5kHz followed by a dip at 7kHz (remember dips are fairly benign subjectively), before recovering and holding out to about 13kHz. The Hunter Elac is smoother through this region, but not quite as extended. Overall though, I'd take either Elac over the Celestion and the Goodmans without hesitation.

* It's worth saying that human hearing is at its most acute in the 2-5kHz region, so these differences really are quite noticeable, especially when you have the sets in front of you. How much the >10kHz bothers you is naturally age-related (and other factors too) - so that's where I'd expect most variation in opinions on a listening panel.
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Old 21st Apr 2017, 7:47 pm   #16
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Default Re: Radio sound quality - some measurements

Yes, the large-magnet Elac only fits the VHF Herald, sadly.

The "Hunter Elac" should fit comfortably in the Sovereign II and VHF Herald, and it's a mystery as to why Hacker never fitted that speaker to those sets. I suspect that one of the reasons was a form of perceived value: the Hunter was the economy set, so it got the cheapest speaker, whereas the Sovereign II was the premium top-of-the-line set, so it got a more expensive speaker. But as we've seen, there's no real correlation between price and performance. And bear in mind that measurements were much harder to do back then - I doubt Hacker had the equipment to do it in-house. So any decisions about sound quality would have been subjective, and at this point we have to wonder if the design team had much of their hearing left up at the 10kHz region... Additionally, perhaps the differences between them were not so obvious with programme material of the time (I'm no expert on '60s music, and rarely listen to it). Back then, AM was what most people were used to, so to those folk, an excess of energy at 4-5kHz was probably quite a novely!

It's interesting to note that the Sovereign IV/RPC1 went for the "Hunter Elac", albeit in 4 ohm guise, and the amplifier was basically the same as well. Simpler than what was used in the Sovereign III, but perfect for the purpose. It's been a while since I listened to a Sovereign IV, but I remember thinking that despite the economies in electronic build quality, it was a fine performer. Must try to remember where my examples are hiding...
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Old 21st Apr 2017, 8:12 pm   #17
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Default Re: Radio sound quality - some measurements

Quote:
Originally Posted by merlinmaxwell View Post
Quote:
I assume the sound quality is down to amp/speaker combination.
More likely the 'speaker box combination (unless you have a really bad amplifier).
This is all rather more nuanced...

Consider a hi-fi speaker, where the box volume, and port tuning if applicable, is very carefully matched to the driver unit. Yes, here the 'speaker-box-combination is important, and will define the sub-200Hz region...

But in the case of these radio sets, the cabinet is neither sealed nor ported. The openings in the cabinet aren't deliberately tuned to resonate at defined frequencies - indeed the box is practically an open baffle. So I don't think the box-speaker combination is as critical here as it would be in a hi-fi speaker.

Moving up the spectrum, the 100-600Hz region is affected by "baffle step". This is a factor of the width of the box: http://sound.whsites.net/bafflestep.htm

These radios are about 11" wide, meaning the BSC frequency is about 400Hz. Look at the graphs - you'll see an distinct bump at 400Hz. This is because there is no baffle-step correction built in. The bass boost from the amplifier is starting at a much higher frequency, so instead of just lifting everything below 400Hz, it's lifting the already-lifted 400Hz along with everything else. As I said above, it's happening to all units, and so there must be a common factor. There it is... Increasing the bass-boost capacitor in the amplifier - to reduce its turnover frequency - would help enormously.

That's one of several similar common mechanisms in the sub-1kHz region. Above that is where we see the differences between the drive units. Though as I've mentioned, some of them might be caused by the grilles or the woodwork behind the grilles. So-called "cavity resonances". At those sorts of frequencies, whether or not you flush-mount a tweeter can have an effect on the response: http://www.zaphaudio.com/mtg-surface.html (the first graph shows a +/-2dB variation in response caused by just the tweeter's 3mm front plate!)

So here, yes, the box is a factor. But actually, what we're mostly hearing is the combination of the drive unit response, and the equalisation built into the amplifier that works with it. Very few speakers are flat and smooth, and most need a bit of help - usually EQ is built into the crossover of decent hi-fi speakers to provide BSC and correction of other anomalies.
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Old 21st Apr 2017, 8:17 pm   #18
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Default Re: Radio sound quality - some measurements

Quote:
Originally Posted by Thyristor View Post
Not overly taken by any of the Hunters; I currenly only own one (an earlyish one) but have owned most of the different varieties over the years.
The earliest examples used a speaker from Fane, then they switched to the Goodmans before settling on the Elac. Additionally, the early non-A models used a different amplifier with different equalisation.

So, depending on how early your current example is, it might well be rather different to the one I've measured today. Next time you get the chance, give an Elac-equipped RP38A another listen

Against that, the Hunter has less FM sensitivity than the other sets, meaning it's not always practical to use mine here. They vary considerably in practice, and despite working on hundreds of them, I still haven't figured out all of the reasons for the variations. One day

So, where did I store my early Hunters...
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Old 21st Apr 2017, 8:36 pm   #19
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Default Re: Radio sound quality - some measurements

Hi Mark.

A very interesting thread. I agree about the excessive range of the bass and treble controls on the Roberts R707, incorrectly adjusted and the set can sound horrible.

It would be good to find out how well the Roberts R600 performs and also the earlier and slightly larger Roberts R700.

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Old 21st Apr 2017, 9:06 pm   #20
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Default Re: Radio sound quality - some measurements

Here's another - the Hacker Aviemore RP78.

This is a late Bournemouth model, fitted with a Philips plastic loudspeaker. Despite appearances, these actually aren't too bad. This set has a pushbutton tone control, so the two curves show the two positions of this switch. You can see that it alters the HF level by about 8dB. Unfortunately, the "up" position is rather too bright, and "down" is too dull. Can't win!

The bass is rather limited, but that's not surprising given the size of the set. The midrange and treble is reasonably smooth and quite well extended, and mostly sounds "crisp and detailed" rather than "annoying". The deep suckout at 2kHz is interesting.

I have other examples of the Aviemore that use different loudspeakers - I'm expecting those to measure rather differently (if I ever find them ).
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