amplifier imbalance

[Murphyv310]

Great Post Mark.
Totally agree with your comments.
I've given advice even on a one to one basis but was totally unaware Rob has a measurement microphone. I also feel that Rob would not be able to interpret the results and how to put the results into practice.
The room is acoustically wrong from what I see of it and if I was in that position I'd buy a decent set of headphones.
For me I'm bowing out of further comment as the thread in my opinion is going round in circles.

[Mark Hennessy]

Totally understand, Trevor - I joined this thread reluctantly, but my posts were attempts at stopping another circular thread. That's the inevitable outcome of subjectivity, sadly.

Your comment about interpreting the results is really important. Rob, please do not take offence at that - as soon as you make measurements in a room, you do have to rely on experience to decide what the room is doing to the results. Of course, none of us are born with that experience - it takes practice. I can help, but ultimately you'll have to put in the legwork.

If nothing else, i can copy/paste bits from handouts that I prepared for work, where we teach engineers about audio measurements, including speakers. We do a practical session that gives people a feeling for what's going on - sadly time constraints mean that it's not terribly in-depth, but as we no longer design our own speakers, that's fine.

In fact, it's better than that, because it shows that the variation in results from professional monitors is really quite surprising. Since we stopped making our own designs, our selection exercises have been purely subjective, on the assumption that monitor speakers will satisfy the really quite fundamental requirement of a basically flat frequency response. But my ears told me differently. My measurement setup simply confirms what I heard, but gives me the data I need to convince people who invest rather too much faith in their hearing.

The more I've learnt over the years about the human hearing system, the less trusting of it I've become. Yes, it's capable of picking out some amazing levels of detail, but it's absolutely hopeless as an absolute measuring tool.

One thing the ear is good at is dealing with reflective rooms - though this ability diminishes with age (the "cocktail party" effect). The measuring mic is not so clever. A technique called "gating" helps, but limits the lowest frequencies you can sensibly measure at the standard 1 metre (typically around 500Hz in a room with an 8 foot ceiling). But that's OK, as the crossover region is the thing you really need to measure during the design process, and bass is relatively easy anyway (modelling software is pretty good). You can take near-field measurements in a room, or put the speaker on top of a 6 foot ladder in the garden if needed. But more on all that later, when the measurement mic is up and running...

Back to the room, if any sort of treatment is out of the question, then just commit "hi-fi heresy" and use the tone controls (or get hold of a graphic equaliser). For non-critical use, it'll be fine. It would be different if you were mixing music, of course. But personally, I really hate lively environments caused by the all too fashionable minimalist interior designs. People underestimate the negative effect bad acoustics can have on their state of mind. This is something that architects take very seriously today, as it can affect footfall - hence profits - in shopping centres, restaurants and the like - there was a really interesting Radio 4 programme about this about a decade ago. I first noticed it about 15-20 years ago when it was subconsciously affecting my choice of local curry-house (we have lots to choose from here!). Definitely not what I'd want in a relaxation room.

But please, no more talk about speaker cables. This is an engineering forum; I am an audio engineer. If you seriously believe that correctly specified audio cables sound significantly different, then you will really struggle to learn about how audio really works. The short-sharp-shock that normally cures people of this is to look at the cables used in broadcast centres and recording studios. It's eye-opening, to say the least!

[Bushbaby]

Mark
Great post and thank you.
My last comment on the internal speaker cable and its nothing to do with, as you have correctly indicated, audio quality etc. In this instance its just a case that the cables connecting the tweeter/driver etc to the crossover wiring is just far too big/thick to solder up correctly hence I am going to replace them with cable that will better to do this with.
Trevor is absolutely correct in his comments that rather than trying to get the upmost audio quality - spend more time listening to the music! at the moment I do feel that I have returned to the venture I had with this over 40 years ago trying to buy the best hifi and it needs to stop. Therefore, I do need to apologise to all.

Best Regards

Rob

[ppppenguin]

As we get older our hearing gets worse. I haven't measured mine (I'm 65) but I know that my left ear is more sensitve than the right and the hearing is clearer in the left, though that's a very subjective judgement. The real surprise came the other day I was running a 405 line monitor. THe 10125Hz whistle was quite loud so I decided to block each ear in turn as a test. I was very surprised that my right ear, the one that's worse for most purposes, can hear 10125Hz clearly while the left ear can hardly hear it at all.

625 line whistle (15625Hz) is now no more than a vague impression that it's happening.

Flat panels are silent, so line whistle is now history except for vintage TV enthusiasts

[Murphyv310]

Hi
My HF response is grim. I cannot hear 10,125hz and struggle at 9khz. I do have aids but in all honesty rarely use them as music sounds very poor. I use headphones and I find they very much work for me. My living room is reasonably good acoustically with carpet, thick curtains and other soft furnishings and it does sound pretty neutral, so Incidentally is the shed, the old stereo 3-3 does a nice job.
Of course Kraftwerk sounds good anywhere!!!

[Mark Hennessy]

As well as apologising in the thread, Rob also PM'd me to apologise. My reply was rambling - it's been a long day - but I'll repeat here one thing I said in the PM: there's no need to apologise

I know that I am a bit "touchy" about some aspects of the audiophile world - there was a long thread about that back in 2020 - and I'm guilty of trying to encourage people to think critically about the claims made, and to apply logic and first principles. Not specifically to tell people what they should buy and believe, but just to help folk to be better informed. Probably a bit irritating, but it comes from a good place.

Anyway, we'll see if we get anywhere with this. It could be that the amplifier needs a bit of an overhaul; it could be that those speakers aren't suitable for that space. Measurements or testing by substitution are possible ways forward. Hopefully we'll get somewhere in due course.

[Diabolical Artificer]

"In this and previous threads, I have stressed the importance of measurements." We all have, we've advised Rob to measure the power supply voltages, to check the DC offset on OP of his amp and other bits and bobs. I admit to getting a bit grumpy with Rob sometimes as he doesn't seem to follow up on any suggestions we put to him. Spending five minutes going over your amp with your DMM will be invaluable Rob, more so than advice and suggestions from this end. A picture of an oscilloscope trace showing the OP of your amp and some voltage readings will mean a hell of a lot more than a description of the sound your setup makes. Lecture over here's some tips.

Get the amp on your bench or table or whatever, power it up and using your DMM on DC volts take readings on both speaker terminals, do the same for AC volts.

Next take the top off the amp, find the power supply PCB and take some DC readings of all the main rails, EG +/- 35v & +/- 15v DC etc. Now do the same with your meter on AC volts. Your looking for the big caps & smaller ones as well as any voltage regulator IC's, these are an indicator of where to check readings. Next check the manual or schematic and compare your readings to those hopefully on the schematic.

Connect an old speaker or better a dummy load to the speaker terminals, attach your scope probes, connect your signal generator to the input, set your sig gen to 1khz @ about 0.5v AC RMS turn on the amp and take pictures of the scope screen. While your at it take pictures of the inside of the amp, decent quality close ups of the PCB's and post here.

Write it all your results down and post here. Lastly Mark made a good point about speaker placement,  see attached pics from a monitor manual I have for a rough idea.

Andy.

[Bushbaby]

I cannot apologise enough for not following up you suggestions/recommendations.
I have now found the microphone which is a Dayton Audio IMM6 - I forgot I had even bought it ages ago!!
The amp will be going into the workshop this week for an evaluation. So this coming week, providing all goes to plan:
- examine the amp output with my headphones (Shure SRH440)
- examine the frequency output
- examine in the amp in workshop.
We have also purchased a rug (we both agreed it was a good idea after all) for the extension too.

Best Regards

[Bushbaby]

Just a quick update.
I have tried the system for the first time using headphones (excellent idea Trevor). I can honestly say the amp is fine and is not imbalanced. The quality of music with the headphones is better than I expected as I did think that the Shure SRH440 headphones weren't the greatest under the sun but I am pleased to say I am mistaken. I did notice that the headphones do not automatically cut out the speakers. It wasn't an issue as I didn't have the volume up too high to I think cause any issues in external sound through the headphones.
I can honestly say that the speakers are still very bright in comparison to the headphones. I will do some frequency checking but wondering if its due to the room or the speakers/crossover that I modified?
Should I just leave the amp alone (my thought are that I should?) although I suppose it wont do any harm to give it a check over?

Best Regards

Rob

[Mark Hennessy]

The Arcam has two sets of outputs - switched and unswitched. They do that for the benefit of those who believe switches in headphone sockets can cause distortion. Not all that unreasonable really - we all know that switched can become tarnished and noisy.

Earlier A60s used DIN sockets with an extra pin, so it depends on which way round you insert the plug. Later versions used 4mm sockets, 2 black and 4 red. Whatever the arrangement, it should be clear from the labelling on the rear panel, and if not, have a look at the instruction manual. All versions can be downloaded FOC from here: https://www.hifiengine.com/manual_librar.../a60.shtml

Incidently, the later manual says that the A60 is happy with 4-8 ohm speakers, which answers the earlier question about 6 ohm speakers. In practice, it's rarely an issue anyway. Please try to understand how impedance is something that varies wildly with frequency, so the quoted value is almost a guess. It's really common for typical "8 ohm" speakers to measure 5 or 6 ohms at certain frequencies. Look here for an example: https://www.stereophile.com/content/miss...asurements (first graph, showing it varying between just under 4 ohms at 250Hz and rising to 16.5 ohms at 2kHz).

I haven't heard the SRH440 headphones, but they seem to be well regarded. The massive spike at 10kHz is a problem - see here: https://www.rtings.com/headphones/reviews/shure/srh-440

I have a pair of Beyer DT770s that are much the same, and while they're OK for checking things during mixing in a live sound environment (because they're closed-back - they also double up as ear defenders!), I wouldn't use them for a critical mix. Or even use them for pleasure. They're a tool, bought for that specific job. The ancient Sennheiser HD480s are much smoother, but being open-back, they're no good for live sound.

Either way, we keep coming back to these damn speakers!

I'm not going to call them Mission 770 - because they are not. I sense that you might not realise just how much difference it makes when you replace the drive units. To get some perspective, have a look here: http://www.zaphaudio.com/8test/compare.html

As you click the make/model numbers above the graph, the graph will change to show the frequency response accordingly. Note that it's 5dB per division.

Try again with 6.5" woofers - there's even more variation: http://www.zaphaudio.com/6.5test/compare.html

Then do the same with tweeters: http://www.zaphaudio.com/tweetermishmash/compare.html

This guy has invested a lot of time and effort, not to mention money, in measuring drive units. He's a really good designer - you can learn a lot from looking around his website.

Hopefully, investing half an hour or so - which is nothing compared to what Zaph has done for the DIY speaker community - will give you a much clearer idea of the challenges here. Your drive units might well fit the holes left by the originals, but bear no relationship at all to the originals.

Of course, that is why the crossover had to be modified. But this mod was done remotely, by well-meaning folk on diyAudio.com, who had no way of verifying their work. This is why measurements are so important.

And in case anyone thinks I've forgotten, we've no idea if the cabinet is correctly tuned for the replacement bass driver. That is easy to check - I've described that before. Here it is: https://golbornevintageradio.co.uk/forum...2#pid86472. This post also contains a lot of what I've said again in this thread - it's all there for the taking, but it does need to be digested and understood (and I remain very willing to help answer your questions, providing - with my "teacher" hat on - they are well-formed questions that demonstrate the progress you're making). Alternatively, you could just pay someone to sort the speakers for you. Or just buy a new pair of something completely different and sell the drivers from the "Missions". I'd pay £33 for them

To assess the affect of acoustics, why not get your other speakers and put them in the space? You know (and like?) how they sound in their normal home, so see what the differences are in the new room. If you can run them on long wires from the existing system, that would be even better (so that you're not running them from different components - basic science: change as few things as possible, eliminate as many variables as possible!).

Or, why not try the "Missions" in your other system? Again, you know (and like?) the sound in that space, right? So swapping out the speakers is surely the quickest way to get a sense of how good or bad your Frankenstein "Missions" actually are. Normally I'd apologise for stating the obvious with these suggestions, but this is the sort of basic thing that I (and some others) aren't sure you've tried yet?

Sorry I've not replied to your email yet - it's been a busy day. Having said that, I think I'd prefer the conversation to be on the forum because others might benefit from it. I get this a lot with emails to my personal website - 90% of them are asking the same questions again and again, and I'm seriously tempted to set up a minimal installation of the forum software on my own site so that people can see my previous answers.

Keep going and, above all, keep it logical. One step at a time. Perhaps keep notes?