Surprised at the amount of engineering in my amp

Just taken the lid of it go give it a spring service, basicaly just dusting

The input switch isn't actually a switch at all, it has a little bycicle breake type cable which then goes into a surfice mount PCB switch thus substantialy reducing the signal path.

I thought this little technique was very clever, I know its very simple but it seems to work pefectly, lets just hope it dosn't snap

 

Lots of amps do that sort of thing. Some pre's mount the input selectors and volume pots right at the back of the chassis (where the signal comes into the amp, by the phonos) and run extension bars through the chassis to the knobs on the front.

 

And some have relays at the back of the amp, and have entirely separate (ie separate mains transformer) power supplies for that gubbins.

 

my pre uses a similar idea to the bar one... to get over using a rubbish motorised volume POT they have used a nice alps pot which is mounted near the back and extended it to the front with a long bar. The bar is in turn controled by a motor mounted at the front of the chassis.. well away from the sound circuitry.

I thought that was quite a nice touch myself

 

My 1992 Naim NAC72 also has an extension bar (well, kind of like a plastic strip thingy) running from the source selector, and so does the MUCH older 32.5, so the idea's not a new one

My Rega Mira 2000's got a VERY strange method of volume control though - uses a normal volume pot, with a motor seperate to it, and also uses the input for this volume control via a click/indent controller which also selects source - a very cool design...

 

The Alps pots with a motor strapped to the back is actually how they sell them as motorised, its not exclusive to that amp.

Can someone explain exactly how a volume pot works? I know how it is wired into the circuit, but how does its internal wiring work? It's simple and I am kind of ashamed to admit I am not sure.

I always assumed it was a simple in line resistor where the signal went through the carbon track and the thinner it was, the less signal got through due to higher resistance. Then I thought that isn't really the best way to do it. Do they actually work by putting the carbon track between the signal and ground so that what you're really controlling is how much signal leaks away and the attenuated output is whatever is left. This way the sound doesn't actually go through the thin carbon track and get degraded at all. Which way is the norm?

 

Different circuits can do it different ways, but generally..

Yes

No - you "tap off" a proportion of the signal between the full signal (at the top of the pot) and nothing (at ground). So the whole of the signal goes through the pot and the proportion of the "whole" value at the wiper is the proportion passed on to the next stage. The sound quailty is therfore affected by the constuction/quality of the reisistive component/material and the quality of the contact onto the pot.

Pot stands for potentiometer, and is one way of implementing a "potential divider". A fixed value potential divider can be implemented with 2 fixed resistors. A pot is a variable potential divider.

No, the "sound" (electrical signal) really does all go through the pot. Whether it gets degraded or not depends on the materials and construction chosen (and your philosophy/beliefs )

 

Tenson: You are absolutely spot on, pots degrade hugely, even the £80 panasonic silver uberpots.
A little trick to do exactly as you say, is pseudo-shunt. That way you are varying the earth potential and not passing the signal right throught the messy pot tracks and wipers. You need a pair of closely matched resistors of whatever flavour you happen to prefer, personally I reckon the vishay boxed things are some of the best Rs you can buy, certainly the best I've heard, £10 a go. Not that much when you consider your passing your entire signal through them. Values between 50K and 100K (50K can rustle a bit when you adjust the volume but don't worry about the noise, no harm done, its just because your varying the earth potential I think- you don't do that while sitting back and listening. 100K can loose a little level though so best to experiment with cheapy maplin Rs first to get the balance before spending cash on gooduns).
Connect these to pin 4 (one on each channel) if your pot is 4 pin or 3 if its 3 pin. Solder the input to the volume control onto the other end of the resistors, then solder the output from the volume pot onto THE SAME pin as the resistors (i.e. 4 or 3).
You then need to solder pins 2 to pins 1 which are the end of the 100K track and earth respectively, then connect these to earth (i.e. run some cable to your star earth point). 4 pin users, leave pins 3, they don't matter.
You should find greatly improved everything sound wise. Some users reported sibilance but I've listened to 3 setups identical to mine (vishays on a panasonic pot) and found none so I can only assume its a component quality issue. Anyway, well worth doing if you can understand that rambling post

 

My soldering is useless, I can even solder a 555 timer to a PCB

My input switch is right at the back as well, just where the signal comes in, the volume pot however is at the front, which must effect the sound quality. My mums amp which is even more basic than mine uses relays for the input selection but it dosn't have the huge elna audio capacitors like mine, and the transformer is some what smaller. Although both have frame types .

My Rotel preamp however just uses switches for input selection.

 

There are far greater effects on the sound. I wouldn't be worried about whether your volume pot is mounted at the front or rear of the case

 

What is your amp , AT, perhaps an internal pic. to show off the engineering?

 

Cyrus amps have the volume pot at the back and the knob at the front transmits remotedly to the pot.

 

Its a Marantz PM6010 OSE.

I should have done it at the time, its back on the shelf and wired up again now though - it takes me a good 30 minutes to rewire it, The amp only costs me £150 brand new which is why I was surprised that any level of real engineering has been done.

The only major cost cutting feature I noticed was the use of a frame transformer rather than a toridial type, but the frame transformer was huge.

 

pseudo shunt, most deffinatly worth a go

 

yes indeed AT, amplifiers, be they transistor or valve are quite wondrous things!!

 

Anex, any chance of a diagram or something? I can't quite follow what you are saying... It sounds like something worth trying though.

Thanks

 

Well I tried doing this 'pseudo shunt'. I have only got cheap 12p a pop resistors in there at the moment but it doesn't sound any worse than using the Alps Blue in a normal configuration! I noticed a bit of sibilance and brightness to start with but it is taming down now. Still a tad there but its only been a few hours I'm sure it will disappear. Also I normally leave my pre-amp on 24/7 so it may be coming from turning it off.

What are peoples favourite resistors and where can I get them? The more neutral the better. By this I mean revealing, not boring. There are sooo many on the vishay website I dont know where to start! What is best for audio?

Actually thinking about it I believe it is now showing the true sound of my system more with the brightness as on my DEQ I added 1db at 16khz and 1.5 at 20khz to bring it closer to a flat response. Now it doesn't need it. I think the Alps rounds off HF a tad.

 

Glad it worked
Like I said in my other post, vishays are awesome, I've not heard a better resistor and nothing else seems to come close. What values did you use in the end? Its about £20 for 2 100Ks from Audiolinks. Sounds alot but everything is running through it so only the best will do!
If you email [email protected] and ask them to mail you a catalogue (.pdf) you can check prices etc. there and order by phone (no website unfortunately).

 

Cheers, I used 91K as suggested in that diagram you sent me. I haven?t tried other values but it sounds just fine so I see no point. What difference would it make?

Surely if it is supposed to have a 100K pot in there then 100K is the best value to use as the rest of the pre-amp was designed around this? Or is it probably best to go for something slightly below 100K because you don?t normal have the volume control turned right down. Either way I think most op-amps as in DAC's and CD players can drive nearly any impedance load so I am not too worried.

I did actually wire it up back to front first time so minimal volume was really full volume... good thing I started with a very quiet CD!

 

D'oh!
The difference can be in the pot response. I found with 100K you can loose a bit of level for some reason (no idea why) but with values such as 50K you can get strange pot behaviour such as noise when you alter the volume level (not a major prob though) or strange response at the bottom of the pot (the level can jump around a little at the lowest settings). Like you say though, 91K should be fine.
BTW vishays will sound awful for quite a while (3-4 weeks) so you need to be patient with them, don't take them out, even when you think they must be done by now- if your thinking that, they aren't done! You'll very much know when they're done, awesome bass, incredibly clear treble, natural mids. Perfect