Radio sound quality - some measurements

[mhennessy]

Originally Posted by Philips210

I can second that. The R700 must be the worst Roberts radio for servicing.

I have found my R700s today, but I've no plans to work on them just yet. I did go inside one about 7 years back, and don't remember it being as bad as the R707.

[Philips210]

Hi Mark.

The biggest difficulty with the R700 is the way the component leads are squashed flat after insertion. I think the R404 used a similar manufacturing technique. This makes it pretty awkward to say the least for servicing. I do however like the R700 which I believe was Roberts first AM/FM transistor set from 1966.

Regards
Symon

[mhennessy]

A few more...

Grundig Music Boy 208, Elite Boy 210, and Elite Boy 600.

I'm especially fond of the smaller sets from Grundig. When working, they have excellent sensitivity and always great sound quality. Hard to restore cosmetically, as we know...

The two older sets have a tone control, which follows Grundig's usual practice of either cutting the bass lift, or cutting the treble. The Elite Boy 600 has a simple 2-position tone button that provides treble cut only.

For the first 2, there are 3 curves, showing tone at min, mid and max. For the latter, the two positions of the tone control switch.

The 4th image shows all 3 sets on the same chart (tone at mid or up). The similarities between them is startling - Grundig clearly had a definite "house style". The Music Boy 208 is blue, Elite Boy 210 is black, and Elite Boy 600 is red.

The Music Boy 208 is the most extended at both ends, but the least smooth overall. The Elite Boy 210 is the smoothest, but least extended at HF. The Elite Boy 600 the best compromise... All very impressive for such small, lightweight sets.

For balance, here's something else:

The Dynatron Elan TP38.

I have a fair few Dynatron sets, but hardly any of them work. Here's one that does, in fetching teak-veneered plywood!

Basically, it's not great. The 4-by-7" Fane loudspeaker has no bass or treble! To get the curve shown, the bass and treble controls were turned up to max. It's fine for AM

[Radio Wrangler]

Your neighbours must now think you've got some strange tropical bird, with all the loud and slow 'whoooop' noises...

David

[Hybrid tellies]

Great work Mark, very interesting results.
I agree what you say about the Hacker Hunter RP38A with the Elac speaker.

My other two best sounding radios are my Bush TR130 which makes listening on AM a pleasure, its very sensitive with a very loud 1w RMS output through its Celestion speaker.
My ITT Tiny Super sounds really nice on AM and FM through its small elliptical Fosters speaker.

[mhennessy]

It's surprisingly not very loud, David, especially if you're trying to keep the volume control within the scope of a loudness tapping. I'm keeping the FM mod down as well, as a lot of these basic discriminators become a quite non-linear as you approach 75kHz. I have a choice of pseudo-random noise or a fast sweep - it only takes about second or two to capture the readings. Averaging is an option, but hardly worth it here...

Simon, I think I've found the box containing my ITT radios. I don't have many, but hopefully a couple that are similar to yours. I remember them having very good FM sensitivity.

It'll be a few days before I can get back to this - other things have cropped up.

[GeoffK]

Re: The Grundig Yacht Boy 210 having excessive loudness, presumably to make it sound more impressive to make its thinner plastic cabinet seem as if it is more like a solid wooden one, the loudness components can easily be removed, either lifting one end of the marked components or taking them out all together. It would be useful if they could be switched in and out but not very practical to fit a switch. Disabling the loudness circuit makes the radio sound more natural, the tone control gives more than adequate range to suit listening tastes, either pushed in for bass or pulled out to put the coupling capacitor in circuit to reduce the bass. The circuit diagram shows the components to lift in red, though the print values aren't very clear. Experiments could be made to either remove the top or bottom part of the loudness control from the volume control, or to substitute different values of resistors and capacitors in the loudness circuit.

[mhennessy]

I posted a clearer version of the schematic of my N210 here: https://www.vintage-radio.net/forum/...554#post371554

The proposed mods would remove all EQ, leaving you with the "raw" response of the loudspeaker+cabinet - which would be severely lacking in bass and perhaps treble too. Some EQ is necessary.

Earlier I described how it actually worked quite well with a "broadcast" that wasn't Optimod'd to within an inch of its life. The problem is not so much the design; it's modern music and broadcasts.

An attenuator ahead of the volume/tone control PCB would preserve the original design intent while rendering the set compatible with today's broadcasts. Alternatively, reducing the gain of the power amp - subject to loop stability - would work.

[GeoffK]

The mod works very well as I have done it to my Yacht Boy after I was disappointed at how it sounded with an extension speaker. After the removal of the components the extension speaker sounded normal, the internal speaker has good bass extension without being overblown and the tone control set to around three quarters gives a natural treble response. Changing the value of 611 could compensate for any change in frequency response but in practice isn't necessary, the radio whether on internal or external speakers sounds more natural, or maybe neutral. Additional attenuation could be fitted to the input of the volume control as the input signal is larger but only noticeable if the volume is turned fully up.

[alanworland]

Been following this post, very interesting and informative.
I have some of the sets tested and can relate to your findings.
I listen to various sets to keep them active and as discovered the R707 is a good performer - well worth the £10 I paid for it!
However one of my favourites is my Tandberg Portable 41, not quite as large as the R707 but with a huge speaker - virtually the biggest that could be accommodated in the well constructed steam bent cabinet. Bass is available down to very low frequencies, and it sounds so fresh and alive - don't suppose you have one for testing!?

Alan

[mhennessy]

Originally Posted by Hetrodyne

The mod works very well as I have done it to my Yacht Boy after I was disappointed at how it sounded with an extension speaker.

Extension speaker? That's rather shifting the goal posts!

This thread is about the performance of complete radio sets. That means the whole thing, from aerial to acoustic output. The equalisation built in to a radio will have been designed to work with the loudspeaker built into the set itself, and that is what we are examining. As soon as you connect some other unspecified extension loudspeaker to a set, then not only are we are off-topic, but we're in a world where there are just too many variables...

Originally Posted by Hetrodyne

Changing the value of 611 could compensate for any change in frequency response...

If you mean C611, it's a coupling capacitor that turns over at about 30Hz - it's not intended to be a filter component. Decreasing it could be a way to reduce bass, but as we'll see, your mods have already reduced the bass excessively. R611 is only in circuit when the tone control is pulled out. Varying that will alter the turnover frequency of the bass-cut high-pass filter.

Either way, this is not the place to get into a debate about the Yacht Boy. I've done some experiments, and will present the data; readers can draw their own conclusions...


Disabling all the loudness filters, as suggested in #47

This involves simply lifting C616 and C617 - there's no need to lift all the components (and cut a track?) suggested in the attachment in #47. Indeed, a double-pole switch could be used if you could find a convenient place to mount it. Note the extra capacitor on the diagram in post #47 - according to my printed manual, that's C613 (4.7n), and only fitted to the 208 model. The diagram I posted earlier is (nearly!) correct for the N210 that I have in front of me here.

The first thing to say is that this mod dramatically changes the "law" of the volume pot, making it much more "fierce" at low settings. The volume control is actually a linear pot, but the loudness circuits "pull down" the level to make it act like a "piecewise" logarithmic pot. Given how much influence these networks have on the frequency response, this was not at all surprising - I was going to suggest this point earlier, but decided to wait until I'd properly checked it out.

For info, I've checked the taps, and they are almost exactly at 1/3 and 2/3. In other words, the 100k track can be thought of as 3 lots of 33k in series. I'm glad about that, because that's the assumption I made when I did the Spice simulation that I posted earlier. This would be an awkward pot to replace!

The first attachment shows the 2 two curves from earlier, with the addition of the plot I've just taken from the modified set:

Blue: Modified - no loudness filters
Red: Volume at 1/3, the lower of the taps
Black: Volume at 2/3, at the upper of the taps

It's surprising how the treble end of the plots overlaps so well. It's tempted to wonder just how much effect the HF boost capacitors (C618 C619) are actually having, though do bear in mind that I adjusted the tone control by ear before taking the plots, so it's not a direct comparison. The Spice simulation does show that the HF effect is relatively mild. I did notice, however, that compared to the diagram I posted (from R&TV Servicing) the HF lift resistors are different - R614 is down from 100k to 33k, and R617 is down from 180k to 100k. The caps are the same. My original service manual agrees with what is in my set. I'll run the simulation again when I get the chance...

To the bass end - here is where the real changes are. Not surprisingly, the blue trace is well-down compared to the others - some 9dB and 18dB respectively at 100Hz. As explained earlier, this is the "raw" response of the speaker in the cabinet, and it's certainly better than many we've seen, but in this mode, it's very "shouty". Yes, there is bass there, but the midrange is really forward, and it's not a comfortable listen at all IMHO.


But while we're at it, can we do better?

We like the black trace; ideally we could do with losing red trace at low volumes. Is this possible? Perhaps, but let's proceed carefully because in a passive circuit like this, all these filters have the possibility to interact.

The easiest thing to try is to remove just C617. On paper, this leaves the effect of the "upper" filter in place at all settings below 2/3 of the volume. What happens in practice?

The second attachment shows the results:

Black: As before, unmodified, volume at 2/3
Red: C617 removed

There are 2 drawbacks to this - first, the volume control is quite fierce - though not as bad as it was when testing Hetrodyne's proposal - and also the HF boost contribution from C619 is missing. As the plot shows, that's a good 3dB at 10kHz - not major, but certainly audible - especially given that this set is a bit lacking in HF generally.

Blue: C617 shorted

Shorting the capacitor is much better. First of all, it allows R618 to continue its role in modifying the volume "law". It's 2k2, so putting that in parallel with the 33k resistance between the end and first tap is clearly going to have a significant effect. With that done, the volume control is much easier to use at low settings (important for a bedside radio).

Next, the HF lift from C619 is back, so the blue and black lines are in much the same place at 10kHz.

At the bottom end, there is 2-3dB less bass than the black curve. That is the loss of the bass lift from C617 that would still be present to an extent even though the volume control had been rotated past the first tap (the effect of a tap doesn't abruptly stop as soon as you rotate past the tap - it gradually tapers away). That's not a bad thing IMO; 3dB less means half the power needed.

In conclusion, the simple act of shorting C617 transforms the N210 into a radio that works brilliantly with today's broadcasts. Listening to it now, it's definitely much better balanced, and the cabinet isn't rattling any more. The only criticism is a lack of "sparkle" because the response falls away so quickly at 10kHz, but I guess that won't bother me for many more years! I could happily listen to this now - which is good, because it's been just a display piece ever since I restored it back in 2010.

So, short C617. It takes no time at all, and is easily reversible if desired. You could even hide a switch somewhere to do it. Or a pot, to give variable bass

Hope that's of interest,

Mark



PS: Mods, if you think it's better to split these posts into a separate thread specifically about the Yacht Boy and these experiments, I'm fine with that...

[mhennessy]

Originally Posted by alanworland

Been following this post, very interesting and informative.
I have some of the sets tested and can relate to your findings.
I listen to various sets to keep them active and as discovered the R707 is a good performer - well worth the £10 I paid for it!
However one of my favourites is my Tandberg Portable 41, not quite as large as the R707 but with a huge speaker - virtually the biggest that could be accommodated in the well constructed steam bent cabinet. Bass is available down to very low frequencies, and it sounds so fresh and alive - don't suppose you have one for testing!?

Hi Alan,

Thanks for the encouragement

I don't own a TP41, but I have fixed a fair few of them. I really like the way they're put together - it's an impressive packaging exercise! From memory, they work well, but I seem to recall a bit of HF harshness. Don't quote me on that - it was a while back. A local friend might still have one in his collection - if so, I'll see if I can borrow it to measure.

Cheers,

Mark

[GeoffK]

The red x wasn't to imply cutting a track, but a disconnect made here, I do not cut tracks when there is no need to. And while C616 and C617 removed will disable the loudness as the other components will give some attenuation to the higher frequency's they where removed. Also any change in C611 would have been larger not smaller, this should have been mentioned. Although I said I was not satisfied with the performance of an extension speaker I was also trying to improve the sound quality of the internal speaker. It was pointed out that experiments could be carried out, either to remove the top or lower tap or to try different values or resistors and capacitors to modify the loudness contour.

[mhennessy]

Originally Posted by Hetrodyne

And while C616 and C617 removed will disable the loudness as the other components will give some attenuation to the higher frequency's they where removed.

How much attenuation will be given to the higher frequencies, please?

Originally Posted by Hetrodyne

Also any change in C611 would have been larger not smaller, this should have been mentioned.

As mentioned, my conservative back-of-the-envelope calculations suggested that C611 turns over at around 30Hz (probably lower in practice). If that's wrong, what is correct frequency? And why would lowering it would be necessary?

Thanks in advance,

Mark

[GeoffK]

I was hoping you would tell others as you have the measuring equipment. The capacitive reactance of the approximate 2nF and 4nF series capacitors will only add a few Kohms in series to the larger series resistors and parallel volume control at around 10Khz giving a small amount of attenuation. I did say I thought it wasn't really necessary to change the network input capacitor.

[mhennessy]

You claimed that my proposal would result in HF loss, so surely it's only fair to ask you to support that assertion up with some numbers?

No measuring equipment needed; just 30 seconds with just a pencil and calculator

[GeoffK]

The Nano farad capacitor reactance in series with the resistors is small compared to the series resistor, the 3.9nF and 2.2nF add around 2 and 4 KOhms at 10KHz reducing with frequency increase but as the tone control is in parallel and is the main determent of the top frequency it makes little difference if these are in or out of circuit, but they will allow an addition parallel path to the tone control as the capacitive reactance falls adding a small addition to the overall HF roll off.

[mhennessy]

How about a rough stab at it? Just a ball-park figure will help; are we talking about 1db, 3dB, 10dB?

[mhennessy]

OK; looks like we're not getting an answer to this. A shame.

Forgive the gentle moan, but I'm sure I'm not the only one who finds it frustrating when claims are casually made with no supporting evidence or data, especially as it takes much longer to debunk the claim than it took to make it in the first place. I did start to explain the theory and calculations, but decided against that in favour of simply producing the data:

Blue: All loudness EQ components removed
Black: Just C616/617 - my simpler alternative

Hetrodyne stated that my approach will result in a loss of treble, but the plots show that this isn't the case.

As we can see, the components I leave in place don't actually change the 10kHz level. They do have have the effect of taming the midrange slightly, which is beneficial, but the largest difference between the two lines is less than 2dB, so it's not a big change.

So, just because the C618/R614 and C619/R617 were providing treble lift before I removed C616/617, it doesn't mean they will provide treble cut when in what amounts to a different circuit.

In the interests of science, I will say that I was very careful with these measurements. I took the time to add wires from the tone/volume PCB to a breadboard, meaning I could change between the two configurations without having to touch the set. So the set, microphone and volume/tone controls were completely untouched between the results. But if in any doubt, run the calculations manually or bung it into Spice.

Really, the question is moot for me because I don't recommend running the set in this way (the bass is ~6-9dB down compared to the midrange and treble) - though if you want to try it, my way is simpler and gives slightly smoother results. As detailed in post #51, simply shorting C617 removes the excess bass, giving a maximally flat overall trend. And now, I shall return my set to that state, having hopefully cleared this up

[Radio Wrangler]

That is a complicated circuit with several time constants close enough together to interact, so a normal manual calculation is either going to be a very involved full analysis, or the usual bode-plot approximations are going to give errors about as big as the amounts of shaping in the midband. The only analysis worth bothering with is to do as Mark says and let Spice loose on it.

On the other hand, the whole thing is all about creating a subjective effect as the volume control is raised, so the final arbiter is whether an owner likes what the sound does at different volume control positions.

Mark's measurements not only show what these things do in concert with the speaker and cabinet effects, he's plotted an awful lot of competitive radios for comparison.

I was never a fan of Grundig, they seemed to do too much messing around. I suppose a bit like the Bose radios of their day, although without that level of marketing bull. Oh crikey! That's torn it... I've mentioned the B-word!