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Old 4th Feb 2019, 9:11 am   #1
Linnovice
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Default Distortion problems

Hi all. At present I run three Aktiv Linn LK 100’s out to a pair of Linn Kaber speakers. These are fed by a Technics SU-C2000 Control/Pre Amp which in turn is fed by a multitude of sources in both digital and analogue. I’ve only been using the Technics for a couple of months. Previously I used a Tascam M50 mixing desk to control all the sources. This worked fine and I never noticed any problems with it. I only changed because it was a massive piece of kit to have in a domestic environment.
It all works well but I’ve noticed that I’m getting a distorted reproduction at a specific range and intensity of signal. It’s top end of the mid range, I’d guess around 8 ~ 12kHz and only on really complicated/intense music. ie. full on choral or orchestral. Solo piano, classical guitar, smooth jazz type, are all very good. The range is good from deep double bass to the highest brushes on cymbals. All is there and reproduced accurately. It is also not subject to volume levels. The distortion is there however loud or soft.
I’ve checked everything. It doesn’t matter from which source, vinyl to digital streaming. All suffer the same. I’ve changed all of the interconnects and speaker cables. Gone through every cable with an Avo to check continuity and resistance all are good. It’s the same in both channels. I’ve had the Technics on the bench and input 20Hz ~ 100kHz (yes, that is 100kHz!) and it outputs a perfect wave right across the range. Likewise the power amps are producing a perfect wave across the ranges set by the Aktiv Cards. I’m now stuffed! Would any of you learned and experienced gents (or ladies) care to offer some suggestions.
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Old 4th Feb 2019, 9:15 am   #2
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Default Re: Distortion problems

Have you checked the speakers?
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Old 4th Feb 2019, 9:31 am   #3
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Default Re: Distortion problems

If you go back to using the Tascam does the problem disappear?
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Old 4th Feb 2019, 10:33 am   #4
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Default Re: Distortion problems

Having proven that the individual pre-amp and power amps are not at fault, I guess it's time to look for a system problem: maybe something is getting overloaded somewhere.

I'd put a scope on the speaker terminals and feed sinewaves into the varous pre-amp inputs to see whether anything bad is happening.

Martin
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Old 4th Feb 2019, 11:05 am   #5
Linnovice
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Default Re: Distortion problems

Yes, checked the speakers. As it’s an Aktiv system there are only driver units in the boxes, no crossovers. The midrange pair were changed last year (due to me blowing on of the originals). All of the individual drivers seem fine. I did have one neoprene sealing washer that was slightly deformed but both sides have now been replaced.
Unfortunately I can’t try again with the M50 as I’ve sold it.

I think possibly there is a system problem and I’m wondering if there could be a mismatch of impedance between the control amp and the power amps. I can’t think of anything else. As the problem is equal on both channels and from various sources it’s a real mystery.

I’ve even tried lying in a dark room but that didn’t help either ��
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Old 4th Feb 2019, 12:06 pm   #6
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Default Re: Distortion problems

I have sent you a private message. It is just possible that you may find the content relevant.
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Old 4th Feb 2019, 1:16 pm   #7
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Default Re: Distortion problems

Quote:
Originally Posted by Linnovice View Post

I think possibly there is a system problem and I’m wondering if there could be a mismatch of impedance between the control amp and the power amps. I can’t think of anything else. As the problem is equal on both channels and from various sources it’s a real mystery.

I’ve even tried lying in a dark room but that didn’t help either ��
You may be on to something there. Impedance mismatches don't usually cause distortion unless an output circuit is required to drive into too low an impedance that's demanding more drive current than the output's designed to provide. I think that may be the case here.

I'm not familiar with this particular Linn kit, but I just looked up the user manual for the Linn Aktiv crossovers and, to my surprise it indicates that it has a rather low input impedance - from 1.6k to 10k ohms depending on the model. In consumer equipment like your Technics pre-amp, a 50 kohm input impedance would be a typical design load. I suspect that the Linn Aktiv may be designed to be driven either from a power amplifier designed to drive a speaker or perhaps a dedicated Linn amplifier.

Your Tascam mixer, being semi-professional gear, will probably be comfortable with a load impedance down to 5k ohm, which could explain why it worked well with the Aktiv crossovers. I would be inclined to connect a pair of power amplifiers between your pre-amp and the Aktiv units. They don't need to be powerful -5W would be ample - all you need to do is to present a lower drive impedance to the Aktiv modules than the Technics pre-amp provides.

That's all a theory, which may be entirely wrong, but I'd give it a try.

Martin
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Old 4th Feb 2019, 2:52 pm   #8
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Default Re: Distortion problems

Sounds like rather a good theory to me.

The fact that both channels are similar makes me suspect it's not something gone faulty.

When you hear problems in music, it can be fairly hard to relate to measurements. You have to be looking in the right place at the right time and in the right way to have a chance of seeing something.

One fall-back I have is to put 8 Ohm load resistors in place of speakers, or speaker drivers in the case of triamped/biamped setups just so that I don't have any expensive whoopsies. Then I sample the sound with a good pair of headphones fed via 100 ohm resistors.

The danger with running test tones into speakers is that test tone sources are usually very flat and amplifiers should be flat, yet the power ratings of midrange and tweetr drivers are progressively smaller than bass drivers. It is very easy to wind up with open circuit voice coils in tweeters.

I also have a handy distortion meter which goes somewhat lower than I can hear and an audio spectrum analyser which cwn let me get a bit more numerate if needed. But if you can hear something via the speakers, the headphones ought to do the business.

You've got a good anonymous nom-de-ploom so the audiophile gang shouldn't get wind of you measuring things... Can wipe zillions off the value of a top-end system.

David
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Old 4th Feb 2019, 3:00 pm   #9
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Default Re: Distortion problems

I also think Martin's explanation is worth serious consideration. A simple buffer amp using something like a TL072 may be all you need.
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Old 4th Feb 2019, 3:07 pm   #10
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Default Re: Distortion problems

At my age, 8-12KHz is right at the top end of my hearing range so for certain I would not be able to hear any harmonic distortion! I am wondering if this is more likely to be a difference in the amplifiers frequency response. Does the distortion level change with volume? Edit is the volume control set to a high level to get anything out of the Linn components? Can you try headphones?
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Old 4th Feb 2019, 3:47 pm   #11
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Default Re: Distortion problems

I took a look at the SU-C2000 service manual earlier today.

In usual Technics fashion, it's basically pretty competently designed, but with a few interesting twists such as their bridge-based Class AA that I don't pretend to fully understand, but am familiar with the basic idea, having first seen it some 30 years back.

We're lucky that it actually makes it very easy to test Martin's theory.

It contains 2 line pre-amp stages - one of which is straight from the volume control, and the other comes after the tone and balance stages. These are switched by a relay controlled by the "Direct" switch, and appear at the output via a rather high 180 ohms.

When in "Direct", the feed comes from a Class AA line amplifier, when not in direct mode, the output is fed from a simple 5480 op-amp buffer.

The same op-amps are used in the "Class AA Flat Amp", but two of them are used in the Class AA mode - the MO is sufficiently different to perhaps show up some differences between the two line output stages.

So in short, is the distortion present in either or both settings of the "Direct" switch?

Honestly, I'd be really surprised if this is the issue. Perhaps the Class AA bridge circuitry is unhappy driving low impedances, but the same circuit is commonly used to drive headphone outputs (my workshop CD player being an example). Looking at the datasheet for the op-amp, it ought to be perfectly happy with 1k8 (1k6+180 ohms). With a 2k load, the THD+N at 1kHz is less than 0.001% above 1.5Vout (it rises below owning to N). It's certainly much better at driving low-impedance loads than a TL07x.

The op-amps run from close to +/-15V in this design, so have plenty of headroom.

I suspect we might be dealing with a fault rather than incompatibility. Don't overlook the possibility of your ears. That's not meant to be flippant - when I turn up a big PA stack, that's exactly the region where I first notice distortion (perhaps more like 3kHz, actually). It goes away when I put earplugs in, and then I can turn up the volume another 10dB or so before it returns. Then I put in my 20dB earplugs, and turn it up some more. You get the idea!

Ultimately, having eliminated this - which honestly isn't as daft as it sounds - then it's a case of dragging the 'scope into the lounge and looking for clipping. Luckily, that's really easy to spot on an analogue 'scope.

Oh, and the last time I had noticeable MF/HF distortion at (relatively!) sane levels, it was a rubbing voice coil on a tweeter (a Vifa D27TG). The best track to isolate it was from Metallica, believe it or not - perhaps the intro to "One" IIRC? Immediately obvious with a sine wave, of course.

But then - re-reading the first post again - you say that it's present at all levels. You mean all settings of the volume control? If so, then it's something before the volume control, making everything above (about op-amps driving low-Z loads and rubbing voicecoils) probably redundant. We need to look before the volume control.

I would start by unplugging any feeds from the Tape outputs. It's possible that badly designed or faulty devices can "load down" the currently playing source if it's also routed to the tape output. All tape outputs have 470R isolation resistors, so that's less likely, but I have seen it happen in the past.

Having done that, that only leaves relays. There's no solid-state switching on this amp - it's all done by relays. They can develop dirty contacts, but it's unlikely to affect all of them (as all sources are equally affected). But everything passes through the tape monitor relay (RL208), so that might be worth a look. The described symptoms don't fit with bad contacts, so this is clutching at strays, but in a logical way. Of course, a visual inspection of the underside of the main PCB would be work a go - this unit is quite old now (thankfully pre-dates lead-free solder though).

Breaking down complex systems is always challenging, but it's the only way. Literally, a CD player into the Technics with nothing else connected apart from a "vanilla" amp working into standard speakers or headphones would be a good first move, then if that's OK, start rebuilding the original system cable by cable until the fault returns.

Good luck,

Mark
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Old 4th Feb 2019, 4:11 pm   #12
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Default Re: Distortion problems

Quote:
These are switched by a relay controlled by the "Direct" switch ...
Daft suggestion perhaps, but might the relay contacts have become oxidised to the point where some low-level diode action was occurring?
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Old 4th Feb 2019, 4:25 pm   #13
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Default Re: Distortion problems

As I suggest, it's certainly a possibility. But both channels equally? It's worth checking, but would I bet my pension on it?

It's quite hard to come up with something that affects both channels equally before the volume control - there's no active circuitry there, so can't blame the power supply. It could be the volume pot, perhaps, and that is a special part with a rotating track, which is interesting in its own right. But again, both channels equally?

The loading effect is possible though. Cassette recorders rarely, if ever, made this mistake, but some devices - perhaps external soundcards - might. The idea being that when not powered, the rails collapse to zero, and the input protection diodes (external or intrinsic) act as clamps to the applied signal at the op-amp input. To get around this, just make sure you've got enough resistance between input sockets and the first op-amp - not a problem in a cassette deck, but in a decent soundcard where you're trying to minimise noise, 10k or so adds rather more thermal noise than you'd like, so you'd have a k or so - perhaps less...

But for that to cause distortion, it depends on the source device having a high-ish Zout and a highish output voltage. Some CD players fit that; many don't. The 470R at the tape outputs will tend to reduce the effect of this mechanism too.

Ultimately, I've seen it happen, but can't say for sure if it's the problem here as there are too many unknowns.

It's a puzzle
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Old 4th Feb 2019, 5:51 pm   #14
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Default Re: Distortion problems

I think trying to rule out the pre-amp might be a good place to start and can be done with basic equipment and no risk to the speakers. Just load the pre-amp with a worse case load (and some representative parallel capacitance) and drive it into clipping at, say, 10KHz and see how close this is to your actual needs.

My guess is that it will prove to be heroic in its abilities but who knows?

I did spend some time chasing down some subtle distortion caused by a dry joint in a power amp. I could hear it but there was absolutely nothing visible on the scope. It measured ~ 0.5% THD at typical listening levels and I was a bit surprised I could hear that but was helped by the fact it only occurred on 1 channel!

dc
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Old 4th Feb 2019, 9:25 pm   #15
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Default Re: Distortion problems

I would be inclined to put a volume control after the Technics pre amp
I think the jargon used is misleading as the normal channel just uses 1 op amp and the tone channel 2 op amps to make up the losses.
The output impedance will below 50 0hms so should drive anything.
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Old 4th Feb 2019, 9:38 pm   #16
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Default Re: Distortion problems

Do any of your sources have variable output level? If so you could connect directly missing out the pre-amp.
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Old 4th Feb 2019, 10:43 pm   #17
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Default Re: Distortion problems

Yes, this is a very good idea. If no distortion is heard, then put the preamp back in circuit with just one source connected and nothing else (apart from the output to the Aktiv of course.) Try to simplify the set up as much as possible to eliminate as many potential causes as possible.
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Old 4th Feb 2019, 10:49 pm   #18
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Quote:
Originally Posted by mhennessy View Post

The loading effect is possible though. Cassette recorders rarely, if ever, made this mistake, but some devices - perhaps external soundcards - might. The idea being that when not powered, the rails collapse to zero, and the input protection diodes (external or intrinsic) act as clamps to the applied signal at the op-amp input. To get around this, just make sure you've got enough resistance between input sockets and the first op-amp - not a problem in a cassette deck, but in a decent soundcard where you're trying to minimise noise, 10k or so adds rather more thermal noise than you'd like, so you'd have a k or so - perhaps less...

But for that to cause distortion, it depends on the source device having a high-ish Zout and a highish output voltage. Some CD players fit that; many don't. The 470R at the tape outputs will tend to reduce the effect of this mechanism too.

Ultimately, I've seen it happen, but can't say for sure if it's the problem here as there are too many unknowns.

It's a puzzle
This is definitely worth checking. I had this exact problem with my Exposure 3010S2 amp. When i connected my Denon solid state recorder to the tape output, the Sony SACD player distorted quite markedly with the recorder powered off. Things get better with it powered on, but not perfect. I no longer use the tape output of the amp!

In a complex set- up, things can interact in unexpected ways. So I would start by simplifying the set up to eliminate as much as possible. Just a simgle source connected to the pre-amp and see what happens then.
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Old 4th Feb 2019, 10:55 pm   #19
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Quote:
Originally Posted by peter_sol View Post
I would be inclined to put a volume control after the Technics pre amp
I think the jargon used is misleading as the normal channel just uses 1 op amp and the tone channel 2 op amps to make up the losses.
The output impedance will below 50 0hms so should drive anything.
The output impedance is basically 180 ohms (R401 and R402). What matters is how much current drive the op-amps have, and how their distortion rises as load impedance falls. I've already checked that - it's very good at driving 2k. Whether it's as good as the venerable 5532 into lower loads is not apparent from the datasheet, but we might be as low as ~2k in this scenario, and the graph shows us there should be nothing to worry about

The "normal" channel uses a pair of op-amps in the Technics "Class AA" configuration. It's not very clear from the way it's drawn, so I've attached a couple of images from a different Technics unit that might make more sense, perhaps.

Also, the primary op-amp is supplemented by a pair of J-FETs. Hardly worth it given the gain is only around 14dB, but Marketing like that sort of thing

The other path uses the the same op-amps as the Class AA stage, and the performance is nearly as good if the specifications are to be believed. The output op-amp has unity gain and is merely a buffer that is necessary after the balance control and mono switch. There is no loss in the tone controls as - despite initial appearances - it is not a passive type with the usual 10-15dB or so of insertion loss. Indeed, that insertion loss is turned into gain because of the way it's in the negative feedback path. Note also that it doesn't invert the polarity of the signal, unlike the conventional active Baxandall. I've seen this setup used around the power amp in several Japanese amps, which in turn simplifies the pre-amp. It's neat
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Old 5th Feb 2019, 4:51 am   #20
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Default Re: Distortion problems

Still try a volume control after the pre amp and see what that does.
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