Thanks AngelP, for confirming my assumptions on the input. Yes, the pot is that black-brownish eyesore with two concentric shafts - wich makes adjusting balance possible but is impossible to turn off or get centered. As soon as a replacement arrives, it will see the might of my hammer 
The 150pF should suffice, given that this amp usually only drives speakers that don't do much over 15kHz and I'm not lusting after a >50kHz -3dB point. I'm not a bat and my ears are not golden.
I was afraid of any DC-bias because of the funky volume pot and because the amps source is essentially a 3.5mm stereo-plug that gets connected to whatever needs amplification. The latter part meant I could never be shure about DC on the input, and I assume a MKP >5 times bigger than what it replaces should do a great job blocking it. It does.
Thank you, anatech, for re-illustrating the importance of a matched input pair. Now I want 2 matched pairs of K170s for my main Denon amp - 50$ in the forum store
. You'll be pleased to hear that it runs twice maximum allowed bias with multiturn pots in daily use since a couple of years, now...
About the multi-turn trimmer: Are Bourns good enough?
The specs say 0.5W (meaning ~14mA of current or 7mA through the wiper, if the specs are to be believed). The resistor in series has 500 ohms more (checking the untouched old pots, over 2,5 times the resistance, actually) and the same dissipation rating as the wiper. Should the wiper fail before the resistor does, I'll have a Class B amp. A "risk" I'm willing to take.
You're right, though - the single-turn-types are more robust. But it's just not needed, here. Not, unless I change the resistor feeding it to a bigger one, at wich point I should be looking into actual wirewound rherostats that cost more than this amp was ever worth...
I did indeed find that the old pots are of good quality. But ease of mind comes through different things for different people. I, for one, can already hear the faint, distant screams from the voodoo people, calling me out for using regular MKPs on the input instead of the unobtanium-in-virgintears kind...
What does actually kind of scare me, dissipation-wise, are the two bando transformers. They're relatively noisy and are the warmest part of the amp. The fact that half of the heatsink of the channel that's nearest to it is blocked by the case cover doesn't help, either. I'm certain some crappy looking holes will help here, thermally as I'm quite shure the reason for this is to keep the holes on the case cover symmetrical (yay, "function follows form", my favourite design-philosophy
) or cost-cutting measures on the case. Who, in their right mind, would block half of the top of an already small heatsink
The 150pF should suffice, given that this amp usually only drives speakers that don't do much over 15kHz and I'm not lusting after a >50kHz -3dB point. I'm not a bat and my ears are not golden.
I was afraid of any DC-bias because of the funky volume pot and because the amps source is essentially a 3.5mm stereo-plug that gets connected to whatever needs amplification. The latter part meant I could never be shure about DC on the input, and I assume a MKP >5 times bigger than what it replaces should do a great job blocking it. It does.
Thank you, anatech, for re-illustrating the importance of a matched input pair. Now I want 2 matched pairs of K170s for my main Denon amp - 50$ in the forum store
. You'll be pleased to hear that it runs twice maximum allowed bias with multiturn pots in daily use since a couple of years, now...About the multi-turn trimmer: Are Bourns good enough?
The specs say 0.5W (meaning ~14mA of current or 7mA through the wiper, if the specs are to be believed). The resistor in series has 500 ohms more (checking the untouched old pots, over 2,5 times the resistance, actually) and the same dissipation rating as the wiper. Should the wiper fail before the resistor does, I'll have a Class B amp. A "risk" I'm willing to take.You're right, though - the single-turn-types are more robust. But it's just not needed, here. Not, unless I change the resistor feeding it to a bigger one, at wich point I should be looking into actual wirewound rherostats that cost more than this amp was ever worth...
I did indeed find that the old pots are of good quality. But ease of mind comes through different things for different people. I, for one, can already hear the faint, distant screams from the voodoo people, calling me out for using regular MKPs on the input instead of the unobtanium-in-virgintears kind...
What does actually kind of scare me, dissipation-wise, are the two bando transformers. They're relatively noisy and are the warmest part of the amp. The fact that half of the heatsink of the channel that's nearest to it is blocked by the case cover doesn't help, either. I'm certain some crappy looking holes will help here, thermally as I'm quite shure the reason for this is to keep the holes on the case cover symmetrical (yay, "function follows form", my favourite design-philosophy
) or cost-cutting measures on the case. Who, in their right mind, would block half of the top of an already small heatsink 
Hi 1210,
So tell me, why exactly are you using multi-turn trimmer resistors again? Because the other Lemmings are?
-Chris
You have not interpreted the spec sheet correctly. The maximum dissipation is across the entire resistance element. It does not indicate that the wiper is supposed to conduct any substantial current.
Excellent choice. Can you please give me the link to the datasheet you are reading from?
You see, there's the rub. Sadly, heat damage is cumulative and indiscriminate. By the time it fails, the damage has been done. I have had to write off several amplifiers and receivers over the years due to extended running at excessive temperatures. If you keep yours cool, then great. Otherwise its a case of once you notice, the damage has been done. The reason for writing those units off? Burned PCB in many areas with foil coming loose. If the item cannot be repaired economically - or reliably, I won't fix it and give the customer the choice. They typically disagree until I show them the damage. Often it just is not economical to replace every electrolytic capacitor and a bunch of transistors when the traces on the hot parts have become un-bonded from the substrate.
The peak current through the bias network can increase with power levels (higher drive) and you would be surprised to see some resistors that have been found burned open. Depends on the design and what is actually going on with the amplifier at the time. All I can say is, just don't be too sure of yourself. All I can do is tell you what I have seen in the past in hopes that you can avoid the same problems.
So tell me, why exactly are you using multi-turn trimmer resistors again? Because the other Lemmings are?
-Chris
Hi Chris
Here's the datasheet for the 3296 Bourns
The datasheet says Wiper: 50% (ACTUAL TR) +-10%. I'm assuming that means ~0.25W for the wiper, then. Hence my 7mA for the wiper. I used 2K instead of 2.2k, btw. Minimum bias is still a couple of turns until 0.1 mV show up on the test points.
That Denon "died" at least twice on me, before I decided I'd have none of it and "got to work". First, a dry solder joint. Okay, easy fix. Second time burned ground traces I didn't notice. Neither did the repair-guy who ripped me off (he just re-soldered some joints here and there and that was it...). It only worked when he showed me because it was already warmed up and the levels were very low. At home, every time I cranked up the volume and as long as it was cold, the distortion was unbearable. That's when I decided to lay hands on it. It was not until then that I even measured and adjusted the bias (wich was at the lower side of what the manual said). The heatsink does now get warm to the touch, but not excessively so. Touching the heatsink indefinetly is not a problem.
I did actually turn the bias down this summer, though. 40°C outside + a lot of bias do not make for a nice room climate...
But enough of that. This thread is about improving Rotels and not Denons or multi- VS single turn pots. And mine desperately needs more safety for the speakers (already on its way here), the amp itself (zobel+inductor, remember?), a better pot and ideally a combination of some mods posted here. The Stage 1 mods are now working like a charm...as long as no speakers are connected on startup
Yes. That and because of "ease of fooling around without fire".So tell me, why exactly are you using multi-turn trimmer resistors again? Because the other Lemmings are?
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Gents, steady on.
Bias trimmers:
I can only follow Chris' crusade against multi-turn pots up to a point.
It is very true that these devices come in varying quality - from great (Bourns, Vishay etc.) to absolute ebay counterfeit rubbish. Trouble is that you can't see what's inside the case.
But... looking at the schematic, yes - the VAS section current is about 6mA, but most of this goes through the bias transistor. The 2k2 bias trimmer sits with its 2k7 series resistor with only 0.6V across both. That is a (centred) wiper current of 160 microamps. Not exactly rheostat territory.
Further, with this bias design, if there was to be a wiper error it would be an open or bad connection and the bias transistor would turn on fully, shutting down the output stage.
So, 1210 if you already have managed to put the multi-turns in, I think that you can safely leave them there. Personally, I see them as a bit of luxury rather than a risk.
Input transistor pair:
Chris is absolutely right - having a balanced input stage design means exactly that - that it should be properly balanced for best performance.
Instead of going through the trouble of sourcing the original 2SA1016s, I would use the well respected Fairchild 2SA992F. I just happen to have a reasonably large inventory of these and if you ask me nicely, I may be persuaded to donate two matched pairs for your project.
However, you have not yet quite ensured a balanced dc load at the input transistor bases, R603/4 must be reduced to match the feedback 33k - or lower if you still want to reduce amp gain. For that, I would instead increase R609/10 to match your reduced R601/2 for better ac balance, but with no preamp stage, you still have no control over the impedances of the attached music sources. So you'll have to roll with what you've got.
Per
Bias trimmers:
I can only follow Chris' crusade against multi-turn pots up to a point.
It is very true that these devices come in varying quality - from great (Bourns, Vishay etc.) to absolute ebay counterfeit rubbish. Trouble is that you can't see what's inside the case.
But... looking at the schematic, yes - the VAS section current is about 6mA, but most of this goes through the bias transistor. The 2k2 bias trimmer sits with its 2k7 series resistor with only 0.6V across both. That is a (centred) wiper current of 160 microamps. Not exactly rheostat territory.
Further, with this bias design, if there was to be a wiper error it would be an open or bad connection and the bias transistor would turn on fully, shutting down the output stage.
So, 1210 if you already have managed to put the multi-turns in, I think that you can safely leave them there. Personally, I see them as a bit of luxury rather than a risk.
Input transistor pair:
Chris is absolutely right - having a balanced input stage design means exactly that - that it should be properly balanced for best performance.
Instead of going through the trouble of sourcing the original 2SA1016s, I would use the well respected Fairchild 2SA992F. I just happen to have a reasonably large inventory of these and if you ask me nicely, I may be persuaded to donate two matched pairs for your project.
However, you have not yet quite ensured a balanced dc load at the input transistor bases, R603/4 must be reduced to match the feedback 33k - or lower if you still want to reduce amp gain. For that, I would instead increase R609/10 to match your reduced R601/2 for better ac balance, but with no preamp stage, you still have no control over the impedances of the attached music sources. So you'll have to roll with what you've got.
Per
Exactly - in most cases quality parts already come in different gain grades and just using the ones with the same grade is good engineering/tech practice, and provides sufficient matching to get better than baseline performance. Other than that in most amplifiers, when we are speaking about drivers and outputs. higher gain parts will peovide somewhat better performance. Why? Because they are as a rule degenerated by emitter resistors. The higher the gain, the more the resistor dictates the actual gain in circuit.
However - I am not a proponent on going overboard. Good matching of outputs and drivers is more than making a simple jig, even with temperature control - this is easily, though sometimes surprisingly seen on a curve tracer. One thing which makes extreme matching in the output stage superfluous is that you can literally NEVER match PNP and NPN parts. Physics does not let you do this. But, you CAN come very close, but for that you really need the capabilities of a tracer, though sometimes, good documentation on the transistors is enough.
For the Rotel, have a careful look at it's output transistors datasheets, specifically the Ib versus Vbe diagram. This should be an exponential, but it isn't - it becomes a line. This shows you that there is a 'built in' emitter resistance in the part, and it is different for the PNP and NPN part. You can get better matching between them by compensating for the difference with different external emitter resistors. Calculating the internal one is not difficult, use the Vbe/Ib slope as a guide. Keep in mind this reduces even harmonics, which may or may sound like an improvement to your ears.
I will have to partly disagree with you on this. Having worked in military and industrial electronics, there is a whole science to controls and trimmers.
There is some truth in what you say but it's not universal. The difference between similar single and multi turn trimmers is really only the worm-gear mechanism, however just like any other part, one really needs to check the datasheet before choosing it for a specific application. Just like not any multiturn pot will be a drop in for a single turn, neither will any single turn one be for another single turn.
The thing is, people often put multi-turn parts instead of single turn, because most suppliers simply do not stock good single turn parts. However, multi-turn ones being much more expensive, they do have a higher quality threshold as is. Still, you can't just use any part. If in doubt and no data is available (which is doubtful, but happens sometimes) use a larger physical size.
Multiturns ARE a problem in production - adjustment is manual, therefore much more expensive than other stages of production, and it's not as straightforward as single turn pots (more time needed, different tools). Also, the price of a multi-turn pot is around 10x that of a sufficiently qualified single turn one, and you know the bean counters WILL see that in your bill of materials and not like it.
It should be noted that NO standard control is really designed to carry (especially DC) current through the wiper contact WHILE IT IS MOVING. Anyone who has had a leaky coupling cap around a volume pot will know why from the noise it makes. Whit trimmers this is really not solved in most cases - they rely on being operated VERY few times in their lifetime. Here is where most of the difference in wiper contacts happens in trimmers.
The other problem is oxidation. In many cases an open wiper is a big problem and in some it is fatal. The same quality single turn and multi turn pot (using the same contact for the wiper, check the datasheet!) will have a lesser seal in the single turn version. However, all seals leak, it's only a matter how much - if the contact is not up to par, it is only a matter of time till it will have problems.
Any turn controls that are to be operated by the user on a (semi) regular basis always work as almost zero current through the wiper. This has more to do with micro-sparking and material migration that actual STATIC wiper current. Even helicoidal wirewound multiturn pots completely sealed will have this problem and WILL eventually fail (their ball contact can seize completely because the surface becomes too pitted for it to freely turn). In fact, the latter should not be used even for static AC current draw as they are quite inductive
Regarding how heavy the contact is, it might be even heavier on a multiturn, but the womgear machanism also gears down the required force so it's really almost impossible to tell. Even more so if the adjustment spindle is sealed.
In short, just like not all single turn trimmers are not created equal, neither are multi-turn ones. Both can be used assuming you chose the right one - and in practise, using a multi-turn is not a measure for finer adjustment, but an attempt to get a better quality part since suppliers rarely carry good single turns.
1210 (is that your real name? - cool), I too am relieved that you decided to reduce the bias current setting for the amp down towards the factory design spec. If you need a winter heater, go for Class A with proper heatsinks - or of course a valve amplifier.
The paltry heatsink inside the Rotel is part of the total design cost minimization - as is the mains transformer, which I painfully experienced recently. Don't deliberately run things constantly too hot - as Chris said, heat degradation is cumulative.
On that note, and with high respect for experience, I had further thoughts on why Chris has apparently seen so many failures with multi-turn pots.
Comments please on the following (and I think quite common) failure scenario:
a) The bias transistor eventually get tired of its life on the (too) hot heatsink and decides to goes open circuit in protest.
b) That makes the driver/output transistors turn on fully until at least one goes into meltdown and short circuits
c) Before the fuses finally blow, a high current has shot through the remaining path through the (multi-turn) pot wiper and frying it.
So, could the poor multi-turn pot in fact be quite innocent, but in the wrong place at the wrong time - yet getting the blame for the disaster?
Per
The paltry heatsink inside the Rotel is part of the total design cost minimization - as is the mains transformer, which I painfully experienced recently. Don't deliberately run things constantly too hot - as Chris said, heat degradation is cumulative.
On that note, and with high respect for experience, I had further thoughts on why Chris has apparently seen so many failures with multi-turn pots.
Comments please on the following (and I think quite common) failure scenario:
a) The bias transistor eventually get tired of its life on the (too) hot heatsink and decides to goes open circuit in protest.
b) That makes the driver/output transistors turn on fully until at least one goes into meltdown and short circuits
c) Before the fuses finally blow, a high current has shot through the remaining path through the (multi-turn) pot wiper and frying it.
So, could the poor multi-turn pot in fact be quite innocent, but in the wrong place at the wrong time - yet getting the blame for the disaster?
Per
Sadly, I'm not named after one the most culture-changing electronic products ever made - the Technics SL-1210. I choose not to post my name here since I've had bad experiences publicly posting my name in forums. Sadly, the world can be quite cruel...
I'm already drooling over those F5T pcbs in the forum shop. Sadly, it costs "a bit" more than just the PCBs to actually build one (or even two) of those.
Don't worry, when I'm messing with bias, I'm always very cautious about the heatsink temperatures. I'm not trying to kill this amp through heat. Quite the contrary - it'll get taller feet and a couple of holes to make the half-blocked heatsink of one channel actually work. As I said, the transformers run warmer than the heatsinks, no matter the bias. The bias also lowers automaticly with more heat, so even if I ever crank it up again, it should be safe.
Doesn't lowering the R603/604 further than R637 (R639 on the BX2), apart from establishing a balanced load, only lower gain by attenuating the signal going in more? That doesn't sound like a better method than just using the volume potAngelP said:
My assumption was that going into the amp with as much signal as possible and having less gain in the amp itself would yield better results/less noise. This could be done more elegantly by increasing feedback through lowering R637 or raising R609/610 (R611/612 on the BX2), as you suggested. I take it that lowering R637 (and R603 accordingly) is not recommended, then?
But you're right. This passive volume control makes it impossible to exactly balance AC load for anything else than a single volume position with a known source impendance. And I will not go through the hassle of soldering up 80 or so resistors on a custom, passive, stepped attenuator to achieve AC balance on the input for every volume setting. Even putting an OP-amp into the input for that would be easier.
Hi Guys,
I just wanted to point one thing out. On the Bourns datasheet there is another link to an applications sheet. It would be good reading for many of the folks here. The direct link is here: http://www.bourns.com/data/global/pdfs/ap_proc.pdf
This sheet is definite in calling the trimmer a potentiometer, or not designed to have substantial current flow through the wiper. All electrical measurements are made with the wiper at one end of the control. The section on the first datasheet was a mechanical spec, not an electrical one. The wrong conclusion is easy to arrive at since those short forms are not well defined.
Having said this, all I can say is that sometimes parts can survive for a long time being used improperly. The document does agree with my training, and I'm sure for many others too, but it could be made more clear as to what they mean rather than simply relying on the definition for "potentiometer".
Yes, using a multi-turn potentiometer has been a sore spot with me. I'm just trying to point out that this popular component change is not recommended as the part is not designed to be used in this manner. It's function pretty much demands that it is adjusted while power is applied and current does flow through it.
-Chris
I just wanted to point one thing out. On the Bourns datasheet there is another link to an applications sheet. It would be good reading for many of the folks here. The direct link is here: http://www.bourns.com/data/global/pdfs/ap_proc.pdf
This sheet is definite in calling the trimmer a potentiometer, or not designed to have substantial current flow through the wiper. All electrical measurements are made with the wiper at one end of the control. The section on the first datasheet was a mechanical spec, not an electrical one. The wrong conclusion is easy to arrive at since those short forms are not well defined.
Having said this, all I can say is that sometimes parts can survive for a long time being used improperly. The document does agree with my training, and I'm sure for many others too, but it could be made more clear as to what they mean rather than simply relying on the definition for "potentiometer".
Yes, using a multi-turn potentiometer has been a sore spot with me. I'm just trying to point out that this popular component change is not recommended as the part is not designed to be used in this manner. It's function pretty much demands that it is adjusted while power is applied and current does flow through it.
-Chris
Hi ilimzn,
Thank you for your comments. They are always insightful.
Matching parts using the jig I designed is done at the tail current the parts are run at, so the parts are matched at the resting point. If they diverge from there, it will not be by much as the parts are supposed to be from the same batch (on tape is better). The jig can make the match so tight that you also have to match the degeneration resistors too. The match does result in a much lower THD reading (by about 1/2 often), and that is really something. If you are going to match some parts, may as well get them really close.
As for complimentary parts, you are absolutely correct. Compliments will never be truly matched. Again, you can get them closer than random selection from a pile of parts. Every little bit helps. As for paralleled output transistors, matching them (looser of course) does also reduce distortion. On Semi has some material on that, but it is a single line comment on a few datasheets. They indicate a 20 dB reduction before feedback is applied. That is a worthwhile goal. I have matched these things almost since I began my electronics career, and other people I've repaired equipment for almost always mention that it sounds better after the repair. For years I didn't know why, but accepted that is was just repaired per the manual. I know why these days, so may as well spread the knowledge.
-Chris
Thank you for your comments. They are always insightful.
Matching parts using the jig I designed is done at the tail current the parts are run at, so the parts are matched at the resting point. If they diverge from there, it will not be by much as the parts are supposed to be from the same batch (on tape is better). The jig can make the match so tight that you also have to match the degeneration resistors too. The match does result in a much lower THD reading (by about 1/2 often), and that is really something. If you are going to match some parts, may as well get them really close.
As for complimentary parts, you are absolutely correct. Compliments will never be truly matched. Again, you can get them closer than random selection from a pile of parts. Every little bit helps. As for paralleled output transistors, matching them (looser of course) does also reduce distortion. On Semi has some material on that, but it is a single line comment on a few datasheets. They indicate a 20 dB reduction before feedback is applied. That is a worthwhile goal. I have matched these things almost since I began my electronics career, and other people I've repaired equipment for almost always mention that it sounds better after the repair. For years I didn't know why, but accepted that is was just repaired per the manual. I know why these days, so may as well spread the knowledge.
-Chris
This is an absolute must. It is fairly simple to derive the necessary currents and voltages for small signal stages and matching things like differential pairs and current mirrors is recommended.
The funny thing is that devices according to JIS (Japanese parts or clones/descendants) were sorted into gain bins from the start (originally there was a colored dot, later the first letter of the color). They are usually specified in 1:2 exponential values (i.e. 100-200, 200-400, 400-800) and were specified wherever it was critical. Pity there was never a good standard for that for US and EU devices, though EU small signal ones had a semblance of it - but nothing like it for power transistors.
Now, power transistors are generally difficult to match, especially MOSFETs (although these days they come with a very tight spread right from the factory if bought from the same batch).
As you mention, matching multiple pairs is beneficial - this is well known and the more important the smaller the value of the emitter resistor. Unfortunately it's not only gain (beta) that must be matched but also Vbe especially is a small quiescent current is sued with fairly small emitter resistors - this sort of thing generates multiple combinations of varying gain at crossover, between each possible pairing (PNP/NPN or Pch/Nch). So, instead of one crossover region, you get multiple smaller crossover like regions around zero crossing, or, simply put, instead of one relatively large discontinuity in the transfer characteristic, you get multiple smaller 'wiggles' in it. So, instead of the signal going through a discontinuity one for each zero crossing generating a higher level of low order harmonics, if it is large enough it will do multiples for each zero crossing, resulting in bursts of high order harmonics though each at a lower level. Still, this is less desirable than the first scenario. Matching multiple output pairs will reduce this problem signifficantly - a hard learned lesson form some recent no-NFB designs I tried.
Hi ilimzn,
My own experiences have taught me that the closer you can get these beta values matched, the lower the distortion will be. Never mind that the amplifier can be more stable as it warms up (DC offsets for example), and they tend to sound better with much less warm-up. So more consistent amplifier characteristics.
My experiments have pointed to the beta as the most important characteristic for power transistor matching. The Vbe is only a few mV between devices, and at higher currents the emitter resistors swamp that contribution out - hopefully those resistors are close in value. At higher output power, the beta takes over as the most dominant factor in current sharing. Thankfully the emitter resistors also improve the effective match as well. It doesn't hurt to select output transistors by beta, then sort within that group for Vbe. You wouldn't do this for any repair, but on really good equipment it is worthwhile. On a nice build for yourself, why not treat yourself to the best amplifier you can make?
We are otherwise in perfect agreement ilimzn.
Best, Chris
Myself included, but a few decades ago.
True, but that isn't close enough. Early Marantz replacement parts were pre-sorted from the service department (for example) with their own bin indications. Input transistors could be ordered matched, which I had done as well.
My own experiences have taught me that the closer you can get these beta values matched, the lower the distortion will be. Never mind that the amplifier can be more stable as it warms up (DC offsets for example), and they tend to sound better with much less warm-up. So more consistent amplifier characteristics.
My experiments have pointed to the beta as the most important characteristic for power transistor matching. The Vbe is only a few mV between devices, and at higher currents the emitter resistors swamp that contribution out - hopefully those resistors are close in value. At higher output power, the beta takes over as the most dominant factor in current sharing. Thankfully the emitter resistors also improve the effective match as well. It doesn't hurt to select output transistors by beta, then sort within that group for Vbe. You wouldn't do this for any repair, but on really good equipment it is worthwhile. On a nice build for yourself, why not treat yourself to the best amplifier you can make?
We are otherwise in perfect agreement ilimzn.
Best, Chris
A little update on my RA-820BX2:
I can't find a jumper to put the R of the RC-filter in on both negative supply rails for the VAS/input stages. Since there are some parts on its way from china (stepped attenuator + speaker protection) and, thanks to a very generous donor [Per, praised be thy name] from across the channel, I'll wait with the cutting of traces for Stage 2+ mods until next year. It also looks like the attenuator I ordered could be too big to fit in the stock location. I might have to put in an axle to relocate the volume attenuator or cut the phono stage out.
I've more or less fixed the blocked heatsink situation by drilling some holes in the cover - it turned out better than expected. The heatsinks now get warm more equally and the transformers don't absorb as much heat from the heatsink that was blocked.
DC offsets at the outputs are currently at -6.8 and -4.9mV when warm. Not bad. And I haven't balanced the DC loads of the (still unmatched and not thermally coupled) input pairs, yet.
This is going to be a nice little amp in ~2 months time
I can't find a jumper to put the R of the RC-filter in on both negative supply rails for the VAS/input stages. Since there are some parts on its way from china (stepped attenuator + speaker protection) and, thanks to a very generous donor [Per, praised be thy name] from across the channel, I'll wait with the cutting of traces for Stage 2+ mods until next year. It also looks like the attenuator I ordered could be too big to fit in the stock location. I might have to put in an axle to relocate the volume attenuator or cut the phono stage out.
I've more or less fixed the blocked heatsink situation by drilling some holes in the cover - it turned out better than expected. The heatsinks now get warm more equally and the transformers don't absorb as much heat from the heatsink that was blocked.
DC offsets at the outputs are currently at -6.8 and -4.9mV when warm. Not bad. And I haven't balanced the DC loads of the (still unmatched and not thermally coupled) input pairs, yet.
This is going to be a nice little amp in ~2 months time
Yes, the compact PCB layout of the 820BX2 looks a bit tricky for mods. Seems like you will have to cut the V- track between C607 and R621, drill two additional holes and mount a "standing" filter R there. Shouldn't be too difficult.
Good news on the heatsink, and please note - it is already a nice little amp. In two months you will have an outstanding little amp
Per
I might have to put in an axle to relocate the volume attenuator or cut the phono stage out.
Blimey, that sounds drastic - are you sure it is worth it - for a volume pot?
Good news on the heatsink, and please note - it is already a nice little amp. In two months you will have an outstanding little amp
Per
Blimey, that sounds drastic - are you sure it is worth it - for a volume pot?1210 said:
Good news on the heatsink, and please note - it is already a nice little amp. In two months you will have an outstanding little amp
Having an amplifier that amplifies both channels to +- <.5dB instead of +- 6 dB sounds like it would be worth it, to me. I could also have some luck an be able to fit the new attenuator without modifications, I don't know, yet, as there were no measurements provided (and I can only estimate it by looking at the sizes of similar style pots). I also don't have a seperate phono-stage, yet, and I only have one turntable (to wich this amp never gets connected, anyway). I also happen to be fascinated by pots that are connected by axles - IMHO it just looks awesome
Although the idea of routing the attenuated input signal through half the case makes my axle-idea less appealing...
Only time will tell what I'll to with the phono-pre-/volume-pot-situation.
The holes helped the heatsinks tremendously, but the transformers still get at least 10°C warmer than the heatsinks. I might put some holes in the case around them to help them stay cool, too. I'm not planning on repeating your "my transformer is thrashed"-mishap, Per
Coming from the PC-overclocking side, I am always very cautious about cooling and temperatures - you can never have too much cooling or check the temperatures too often.
Is there anything I can do that might help take some audible humm and heating out of the transformers?
And there's still the issue of too much gain. I don't need 71x - half that would be plenty. Could lowering R639 from 33K to 15-22K (and matching R601+603 to the feedback resistors) work without running the risk of oscillation or greatly reduced stability?
Lowering these resistors also has the effect of reduced AC impendance imbalances for the input pair as the volume pot would have a higher setting/less series resistance for the same volume and it's base value less impact on the impendance imbalances the input pair "sees".
I'm also still quite sceptic about the grey powercord that features a 3 wire schuko plug with ground-connection. All the pictures and schematics I've looked at seem to indicate that there should be a (black) 2 wire power connection to this amp, not a grey 3 wire "general purpose" schuko from 1975. Can somebody back me up on this?
Any ground loop that can be avoided is one less hummingbird to worry about...or, in the case of my pc, definetly heard (an issue I haven't battled, yet).
anatech, as you seem to be a very strong opponent of multi-turn pots for bias setting, are you willing to sacrifice one (or more) by testing it's limits and start a thread about the results? Judging from the back-and-forth in this thread, this seems like an interesting and controversial topic about a very poular "missuse" of a safety related part.
Thanks, everyone, for your contributions to this thread, so far. This is fascinating and insightfull stuff. As a treat, here's a semi-related youtube channel I found funny.
Hopefully, someone will answer my remaining questions without me having to resort to "try-and-error".
Having an amplifier that amplifies both channels to +- <.5dB instead of +- 6 dB sounds like it would be worth it, to me.
Well, if your original pot gives 6dB difference between channels, it definitely deserves the full force of your sledgehammer.
I measured the standard Alps stereo pot which I had panned to go into the 820BX2 - as and when I get round to do that project:
Yes, the absolute value of the two tracks of the stereo pot are different (4.3%) but the attenuation (in %) in three wiper positions were: 21.5/21.8%, 42.7/43.2% and 76.4/76.6%. Even with the high gain of the BX2, I doubt that anyone could ever hear any inter-channel volume difference.
But of course, the (metal film?) resistors in the stepped attenuator will be less noisy than the standard pot carbon tracks.
Anyway, what you have done so far and your plans for reducing the gain of the amp should not give you any stability issues - mind you, this is probably the best overall way of protecting your speakers. On that note, somewhere you said that you are driving high efficient speakers that does not go above 17kHz.
Am I right in guessing that they may be full range 8" driver, horn loaded beasts?
Per
Well, if your original pot gives 6dB difference between channels, it definitely deserves the full force of your sledgehammer.
I measured the standard Alps stereo pot which I had panned to go into the 820BX2 - as and when I get round to do that project:
Yes, the absolute value of the two tracks of the stereo pot are different (4.3%) but the attenuation (in %) in three wiper positions were: 21.5/21.8%, 42.7/43.2% and 76.4/76.6%. Even with the high gain of the BX2, I doubt that anyone could ever hear any inter-channel volume difference.
But of course, the (metal film?) resistors in the stepped attenuator will be less noisy than the standard pot carbon tracks.
Anyway, what you have done so far and your plans for reducing the gain of the amp should not give you any stability issues - mind you, this is probably the best overall way of protecting your speakers. On that note, somewhere you said that you are driving high efficient speakers that does not go above 17kHz.
Am I right in guessing that they may be full range 8" driver, horn loaded beasts?
Per
For the first quarter turn, there's an absolute differnece from 70 to 150% between channels. The bigger one is also changing between channels. Both ends of the wiper and the carbon tracks, however, meassure the same. The imbalances are clearly audible.
The defective pot now gave me the excuse to get a stepped attuator. I like things that do 'click'.
Yes, the new ones will be metal film. Less noise is always welcome and the idea behind going stepped. I could only find a 25KA equivalent, but that won't matter much, here.
Reassuring to hear that. I'll move forward, then.
Yes, they are literally called The Beast (Das Viech). They are ~150L big (on the outside) and back-hornloaded. Mine have a 4 ohm Sica LP208.38/426 broadband chassis. Not the most refined speakers, but they shure can make fun. Before the mods, they seemed not to like the Rotel much and lacked punch. Now they play like they should - the more volume they are allowed to produce the better they sound
Hi 1210,
Did you follow the link I provided and actually read the paper from the manufacturer? This is what you should be doing whenever you use a part you are not familiar with, and more so when the original part is being substituted.
There is no one best part as a blanket statement. Part characteristics fall within the family of parts. These families have uses they are best at while other part families have applications they suit better. To be good at performing modifications, you have to know the characteristics of the parts you are using, much like an aircraft mechanic knows what bolts to use, or a marine mechanic who knows what parts are suited to what they do. So it goes with parts in electronics. Metal film resistors are not the best for all situations, nor are single turn trimming resistors. Same goes for multi-turn trimming resistors. Conductive plastic, carbon, cermet and wire-wound resistors all have uses they are the best part for. This is also true of capacitors and semiconductors. The folks who picked the multi-turn potentiometer to replace a carbon trimming pot didn't have any clue as to what the proper uses where for these devices. The basic operation in the circuit makes the carbon or cermet trimmer the proper part types. It excludes multi-turn controls in case you are wondering.
Once you have read the link, look up the term "potentiometer". Understand what the part was designed to do. Examine the schematic to see if the part is a potentiometer, or as a rheostat. Can you get away using this pot(entiometer) this way? Maybe. Maybe not in a few years. All I know is that this part of the work has not been done correctly, and that is all I need to know. Part of my technical training consisted of this question exactly. What part is suitable for the following job? Understanding the differences between part types is extremely important.
So, I have pointed the way and spelled out what the problem is. It's not up to me to waste my time to prove to myself what I already know. It's up to you, and anyone following along, to educate themselves. I can only hope a couple folks can make the correct choice in parts for replacement. One thing that won't happen is the popular internet myth that multi-turn pots are "better" than the single turn models can be halted. But then, CDs supposedly had better bass when the rim was inked green. How about the green LEDs inside the CD bay? Those took a long time to die even though they were clearly not true.
Best, Chris
Is that a mechanical hum that you hear with the volume at minimum out of the chassis?
I have not only seen enough proof over the years, but having understanding what the terms mean, it seems to be pretty darned clear to me. I have no doubts about this what-so-ever. None. All I can do is to attempt to educate people where errors are being made. Past that, they will do what they will do.
Did you follow the link I provided and actually read the paper from the manufacturer? This is what you should be doing whenever you use a part you are not familiar with, and more so when the original part is being substituted.
There is no one best part as a blanket statement. Part characteristics fall within the family of parts. These families have uses they are best at while other part families have applications they suit better. To be good at performing modifications, you have to know the characteristics of the parts you are using, much like an aircraft mechanic knows what bolts to use, or a marine mechanic who knows what parts are suited to what they do. So it goes with parts in electronics. Metal film resistors are not the best for all situations, nor are single turn trimming resistors. Same goes for multi-turn trimming resistors. Conductive plastic, carbon, cermet and wire-wound resistors all have uses they are the best part for. This is also true of capacitors and semiconductors. The folks who picked the multi-turn potentiometer to replace a carbon trimming pot didn't have any clue as to what the proper uses where for these devices. The basic operation in the circuit makes the carbon or cermet trimmer the proper part types. It excludes multi-turn controls in case you are wondering.
Once you have read the link, look up the term "potentiometer". Understand what the part was designed to do. Examine the schematic to see if the part is a potentiometer, or as a rheostat. Can you get away using this pot(entiometer) this way? Maybe. Maybe not in a few years. All I know is that this part of the work has not been done correctly, and that is all I need to know. Part of my technical training consisted of this question exactly. What part is suitable for the following job? Understanding the differences between part types is extremely important.
So, I have pointed the way and spelled out what the problem is. It's not up to me to waste my time to prove to myself what I already know. It's up to you, and anyone following along, to educate themselves. I can only hope a couple folks can make the correct choice in parts for replacement. One thing that won't happen is the popular internet myth that multi-turn pots are "better" than the single turn models can be halted. But then, CDs supposedly had better bass when the rim was inked green. How about the green LEDs inside the CD bay? Those took a long time to die even though they were clearly not true.
Best, Chris
Hi Chris,
I'll see what I can do to make the primary side a little safer. As is, threre is no fuse, an underrated (125V AC) capacitor over the on/off switch and no filter.
I've opened pandora's box, haven't I?
Yes, it is.
I'll see what I can do to make the primary side a little safer. As is, threre is no fuse, an underrated (125V AC) capacitor over the on/off switch and no filter.
I've opened pandora's box, haven't I?
Gents,
Thank God that things have calmed down a little so even Captain Slow (i.e. Mr. Overworked) here can keep up. I have looked through 1210's past questions and my replies are below.
I've got one (1) such cable for testing – and half a BMW in the garage. (Actually, I haven't even got half a beamer, I just coudn't resist the pun.)
Buying quality hurts your pocket for a short while, buying cheap stuff hurts you every time you use it.
Or, keep it high - and live with and enjoy the warm(ish) transformer. With your speakers, I guess that you will rarely be playing with more than 1-2W before the neighbours come crashing in to warm your ears?
Not that I doubt that you can do it properly, but you are venturing into fire or electrical shock insurance liability territory. And these financial guys decide culpability solely on the basis of officially stamped certificates – not however safely and well things were actually done.
PS.
Reading your latest post only makes these concerns worse. That (125Vac???) capacitor across the power switch sits there to quench any switch contact spark or arching when you power the amp on or off. It is not there to go up in smoke and should be voltage rated much higher for 240Vac countries, in fact I don't think that it is even ok for 120Vac territories.
Perhaps someone before you replaced it, but as you are the most recent person to have your fingers in the amp's circuits - guess who has to prove their innocence? Pandora's Box indeed.
I would definitely replace it with a proper kV rated ceramic safety capacitor, but I may be overreacting?
Time to call in the cavalry of experienced guys on this blog for advice.
Per
Thank God that things have calmed down a little so even Captain Slow (i.e. Mr. Overworked) here can keep up. I have looked through 1210's past questions and my replies are below.
are a .1µF/10ohm zobel and a more or less random inductor that "looks the right size" a good idea, here? If so, what wattage resistor be enough in the zobel and inside the inductor?
Yes, Zobels are a good idea to stop instability from excessive inductive loading. The mostly used (and working) solution is indeed the series of 0.1uF and 10ohm (a 1 or 2W type would probably do ok in this low power amp) and should be mounted from the junction between the power transistor emitters to ground, I.e. before the output inductors – not at the speaker terminals. Mind you, your 0.22ohm wirewound output transistor will actually to some degree act as a dampened output inductor to isolate against excessive capacitive loads, although your single full-range driver should not be a problem in that respect – unless you already have pawned your BMW to invest in Gold Plated Silver Threaded Double Braided Two Colour Chromated Capacitance Limited Edition Monster Speaker Cables.
I've got one (1) such cable for testing – and half a BMW in the garage. (Actually, I haven't even got half a beamer, I just coudn't resist the pun.
)There are other, much more important and usefull things I'd like to get before I aquire a plethora of parts to use in later projects: a scope and a LC-meter. Maybe even a non-crappy multimeter (ideally with a built-in transistor-tester).
Absolutely. Without a scope (even a basic one) you are blind. But - without a decent multimeter you should probably best keep your hands in your pockets on all things electronic. I managed without an LC meter for many years, but my very first instrument purchase (back when the Queen was still a mere lad) was one of the first Fluke 87. It totally broke the bank at the time, but it has saved my bacon on so many of my early (and later) projects. The display is now intermittently a bit dodgy, but I still use it and rely on it as an old friend. And as they say:
Buying quality hurts your pocket for a short while, buying cheap stuff hurts you every time you use it.
Doesn't lowering the R603/604 further than R637 (R639 on the BX2), apart from establishing a balanced load, only lower gain by attenuating the signal going in more? That doesn't sound like a better method than just using the volume pot
Yes/No. Sorry, I probably wasn't quite clear. With properly paired transistors installed, you then need to ensure that their base dc current loads are the same. Thus, in this case, R603 to have the same value as R637. The ac stuff comes later, and in the 820BX2 case, you'll just have to do what you can on this.
Is there anything I can do that might help take some audible humm and heating out of the transformers?
As Chris has advised, first find out if it is mechanical or electronic. Your planned mods should take care of most of the electronic hum. As for the heat, well - bring down the bias to factory specs. Iron core transformers have internal losses that turns into heat.
Or, keep it high - and live with and enjoy the warm(ish) transformer. With your speakers, I guess that you will rarely be playing with more than 1-2W before the neighbours come crashing in to warm your ears?
I'm also still quite sceptic about the grey powercord that features a 3 wire schuko plug with ground-connection. All the pictures and schematics I've looked at seem to indicate that there should be a (black) 2 wire power connection to this amp, not a grey 3 wire "general purpose" schuko from 1975. Can somebody back me up on this?
The RA-820BX2 was designed as a double insulated device with no earthing of the chassis. In the past, some local authorities' regulations required that to be changed to “grounded earth” before sale – it basically makes no difference to the amp's function. And the Rotels came with both black and grey power cables. So, if I were you with (at least I assume) limited or no qualifications in mains voltage safety requirements (safe routing, soldering, isolation, clamping, distances, etc.) - I would leave it as it is.
Not that I doubt that you can do it properly, but you are venturing into fire or electrical shock insurance liability territory. And these financial guys decide culpability solely on the basis of officially stamped certificates – not however safely and well things were actually done.
PS.
Reading your latest post only makes these concerns worse. That (125Vac???) capacitor across the power switch sits there to quench any switch contact spark or arching when you power the amp on or off. It is not there to go up in smoke and should be voltage rated much higher for 240Vac countries, in fact I don't think that it is even ok for 120Vac territories.
Perhaps someone before you replaced it, but as you are the most recent person to have your fingers in the amp's circuits - guess who has to prove their innocence? Pandora's Box indeed.
I would definitely replace it with a proper kV rated ceramic safety capacitor, but I may be overreacting?
Time to call in the cavalry of experienced guys on this blog for advice.
Per
Thanks for the advices. As is, the amp and I are both off the primaries.
Time to pawn the cable and get a beater-beamer, thenI've got one (1) such cable for testing – and half a BMW in the garage. (Actually, I haven't even got half a beamer, I just coudn't resist the pun.
You're right - thanks to the good efficiency of the speakers, the amp rarely has to drive more than a watt continously. It takes over an hour for them to actually get warm at idle or normal operation, so I'm not too worried. Still, I'll adress the issue with a couple more tiny holes to aid ventilation (or accomplish any to speak of) without sacrificing safety.
About the capacitor on the switch:
I'm aware it's there for the spark and switch-ringing, but since it's on the primary it must be a X1/2 or Y1/2 class >250V AC rated one. It can be either ceramic or plastic/paper-film-type, but the proper class and voltage rating is what's important here to ensure insurance liability issues (aside from my lack of papers, knowledge, skill, tools, etc.). The current Y-class MP one is fine for the spark, but not ok for use on my 230V mains. No, you are not overreacting
At least I can identify when there's something seriously fishy going on. I must like fish.