Power Supply Design for a solid state amplifier

[elaar]

Hi, I was hoping someone wouldn't mind looking at this design and commenting on it's suitability.

I'm making a 140w/8ohm amplifier (Randy Sloanes favourite design in his book using L-Mosfets).

I don't need to say much about it because i've written quite a bit on the design pic.
This is the first time i have built a psu and so am a bit bewildered, I have made this design by basically copying from about 4 different designs I have studied.
Things i was wondering include are the capacitors the best performance values, and what type of capacitor should each one be? Have I also missed out any components that could improve the filtering? Should the capacitors be bypassed, should the diodes have bypass caps and with a 500VA transformer is soft start recommended?

Thank you to those that take the time to look at it and comment.

Thanks,
Andy

 

[pinkmouse]

The ground from the caps should go straight to the chassis ground, as should your speaker returns. Save your HQ ground for signal levels.

 

[elaar]

Ahh okay, thanks for the input I will change the design.

I also meant to ask, I have quite alot of copper sheeting, and as the case isn't that large and the toroidal is large, I was firstly going to put the toroidal into an old pc psu case, and use the copper shielding to make a faraday cage for the amplifier pcbs. Should the copper cases then have a route to ground through a cermaic capacitor? (I have seen this done in some amplifiers).

Cheers,
Andy

 

[richie00boy]

In a slight contrast to Pinkmouse, I advise you use the centre of the caps as your star point, so each bridge has its own connection to the caps 0V, not shared like you have drawn it. Then a wire off this goes to chassis. All your speaker returns go to the star point not the chassis. The amp board will have a connection also to the star point which you will then take input screen/ground from, and any other 0V references on the board. You might wish to also have a 'dirty 0V' trace on the board which uses another wire back to the star point, for zobel and decoupling networks.

You need another cap across the other switch. I would use 4.7nF Y-rated caps for this purpose as they fail safe.

Your input and output sockets should be isolated from the case.

I would not bypass very large caps with very small ones as this creates resonant circuits.

A soft start is essential for 500VA, usually over 300VA you would use one.

I would personally put the fuses in the amp rails not before the rectifiers. Thus each amp is fused safely then. If there is a problem with the bridge or caps the primary fuse will blow, if sized correctly and protected with a soft start.

Up to you on the faraday cage, I'd be surprised if you needed it or to even put the thing in a PC PSU box, all hassle for nothing IMO.

 

[AndrewT]

Hi,
just to offer yet another option on your PSU.

Fit a soft start.
F1 should be in the Live connection from the mains. Reduce the fuse rating to as low as will allow the amp to start up and run at full power. It may blow after a week or two due to fatigue from the continual reheating. If so go up one rating i.e from T3.1A to T4A.
Add a second cap across the 2pole mains switch or fit real RC snubbers.
500VA will support about 300W maximum. Is your amp being designed to suit 4ohm speakers?
Move F2 & F3 to after the smoothing caps. F5A in the supply rails will allow about 10Apk into your speakers. Are you designing for 4ohm speakers? F3.1A (about 50Vpk into 8r) will better suit 8ohm speakers.
Single rectifiers perform just as well as dual rectifiers. Less heating with a dual but more voltage loss.
16.6mF smoothing caps in combination with 8ohm speakers (RC=133mS) give a low frequency roll off -3db at 1.2Hz. This should be about one octave lower than the input roll off frequency and about half an octave below the NFB roll off frequency. Leaving a useable LF -3db = 2.4Hz and -1db =12Hz.
If you use 4ohm speakers all these frequencies should be doubled or you can double the smoothing caps to 30mF to maintain RC=120mS.
Personally I would use 180mS for a bass amp or wideband amp.
The PCB 100nF should be located at the highest current users on the amp PCB to bypass current glitches to ground along the VERY shortest routes.
Consider adding an interference suppression cap from live to neutral, Xrated 47nF to 220nF and surge absorber (or parallel absorbers) to suit your maximum mains voltage.
Consider adding interference suppression from live to ground and neutral to ground, Yrated 2n2F to 47nF.

Keep the safety earth and make it PERMANENT. No option!
Keep the disconnecting network from safety earth to Central Star Ground.
Consider adding a ground lift switch in parallel to the other components. You will probably never need it.
Connect your PSU common direct to Central Star Ground. Do not move the Central Star Ground onto the PSU common. It can be adjacent if it suits your PSU and amp locations (see later advice).
Connect Amp PCB power ground to Central Star Ground.
Connect all dirty returns on the PCB to the power ground. These are decoupling caps, Thiel network or Zobel if not located on the speaker terminals and bypass caps.
Connect all clean returns on the amp PCB to the signal ground.
Connect PCB signal ground to PCB power ground using 10r.
Connect RCA input return to PCB signal ground. RCA return should be isolated from chassis.
Connect signal ground to Central Star Ground.
Connect speaker return direct to Central Star Ground.

Connection advice:-
Keep the PSU rectifier/s flow and return, transformer feed and centre tap, smoothing cap flow and return as compact as possible or alternatively keep the loop area in each circuit small by combining wires or twisting. Run a short thick wire from a PSU common to the Central Star Ground , alternatively connect a bolt through the the PSU commons and nut it down tight. On the OTHER SIDE of the nut attach all the high current returns and attach a further nut. Finally attach the signal returns to the bolt and nut tight. The bolt and intervening nuts act as VERY SHORT wires but keep the high currents of the charging side separate from the power returns and the next nut keeps the high currents of the power returns separate from the signal ground reference. Any other dirty returns (relays etc) can be conneted to the wire from CSG to the disconnecting network

 

[richie00boy]

Lots of good advice from Andrew, with my only disagreement being this:

Single rectifiers perform just as well as dual rectifiers. Less heating with a dual but more voltage loss.

Single rectifiers can perform well, but they can never match a dual setup. In a high power setup (anything over 300VA I would say) then I would always use dual rectifiers because of their advantages. The load up the transformer more efficiently (so it runs cooler/you can squeeze more power out of it) but most importantly they allow individual returns and a true star ground, which will result in a higher quality ground.

 

[elaar]

Hi, many thanks for all of the great replies, they're very helpful.

I'll draw up a modified diagram showing in detail how i think all of the common and signal grounds should connect up from what you've said (I've had to read some of the replies multiple times and I think i understand it).

The design in question uses the (now a bit tricky to get hold of ) 2SJ162/2SK1058 L Mosfets, this can do 140w(@8ohms) with 55v rails and 200w(@8ohms) with 68v rails witout any circuit alterations.

I'm aiming at the 140w with 55rails, because:
a) I'm using this as a learning exercise and have no need for a very powerful amplifier (I already have 2 huge power amps), infact I would have been happy with 60w amplifier, but...
b) This is sloanes favourite amplifier and so in my eyes is a good one to do.
c) I already have a 40v+40v 500VA toroidal.

This means I should get a fraction under 60v (59.8v roughly) with no load, which is good because I have 8 good 10000uF capacitors already which are only rated at 63v. They won't have a particularly long life with that high a voltage but 100v ones would be too expensive for me currently.

AndrewT: that is one of the reasons why I have used dual rectifiers, just to purposely drop the voltage that little bit more. Also to answer your question about whether it will be used for 4ohms, the answer is probably not. I rarely use 4 ohm speakers, and even in the off chance I do like i said the amplifier will never be used at high volumes so i think it would be okay.

When it comes to soft starts, I was wondering what method people generally design? thyristor soft-start with an output muting relay, or paralleled power resistors and relays? I was thinking of incorporating a small 30-50VA transformer into the case to run auxillary circuits, maybe opamps, clip detection circuits etc.. What if I connect that transformer directly to the power switch and use power from that to activate the relay (maybe with the use of a 555) which will short paralleled resistors to the 500VA transformer?

Hopefully you won't mind having a quick look at the grounding picture/modified circuit I will post a bit later to make sure I have understood all of your advice correctly.

Many thanks,
Andy

PS sorry, the picture should not include F2+3, they are present on the amplifier pcbs, so that's that part sorted.

 

[richie00boy]

Soft start by relay and resistors, powered off aux supply should be fine. A simple RC charging network and transistor would do the job.

 

[jackinnj]

I would not bypass very large caps with very small ones as this creates resonant circuits.

creates a circuit which will resonate if there is a sharp di/dt -- as in a digital circuit. analog is a bit different and there some things definitely to be avoided --

the bypass capacitors across the reservoir cap prevent radiated EMI (from the rectifiers) and inductively coupled EMI and RFI (from the line connection) from making its way onto the positive and negative rails.

 

[Shredly]

I would regulate the power supply; 140W at 40V should be well under 4A, even with 20W of overhead dissipation, and Pedja Rogic's regulated design here will actually give better performance than all those huge capacitors as well as giving you 5A to play with, which means your PSU would be capable of delivering 5A on each rail, for a total output of 400W to power your 140W RMS continuous output amplifier, which surely will peak at under 300W; thus giving you a safety factor of 33%. With a 500VA transformer, you'll have plenty of safety factor there as well, 25%, and you can probably save some money and time using a regulator instead of trying to use mondo caps to filter the ripple out; those big caps are expensive, and the LM338s are about $5 US each, compared to probably $15 or more for those great big 10,000uF caps. ST makes the 338 too, if you prefer European. In addition, the LM338 regulator will probably do a better job than caps; the ripple rejection ratio is 75dB with a $2 tantalum 10uF cap bypassing the ADJ terminal to the local negative; if greater rejection is desired, you can go as high as 20uF, but National Semiconductor states that additional rejection is minimal above 20uF. You'll need to adjust the value of the 2.2K resistors from the ADJ terminals of the 338s to the local negative, to get the voltage you need. In case it's not clear, by "local negative," I mean ground on the positive half, and negative on the negative half. You should have a 3300uF or 4700uF cap to provide the peak voltage to the regulator, where you now have two 3300uF and a 10000uF.

Here are the design considerations for those resistors:

Assuming your toroid puts out +/- 40Vrms, and has two secondary windings, both of which are normal (output measured RMS, and providing two secondaries) for toroids, you should be getting 56.57VP (Vrms x 2^0.5), so 55VDC might be a bit high; you would have the dropout voltage of the regulators to deal with, and the drops across the regulator diodes. The dropout is 1.3V maximum, and the diodes will drop 0.4V to 0.7V depending on the type. You'll most definitely want to check to make sure that there is adequate voltage drop (1.3V absolute minimum, and many designers would recommend at least twice this, in case of temporary voltage drops from the mains) across the regulators; if there is not, they can drop out and the ripple rejection will be defeated. If you design with 2.6V, your maximum conservatively designed value will be (56.57 - ((2 x 0.7) + (2 x 1.3))) = 52.87VDC. I would recommend you shoot for 52V, which will slightly reduce your power output; you'll get an output of 132.4W RMS continuous. Your drop across the regulators is now (2.6 + .87) = 3.47V. Dropping this 3.47V, and sending the masiumu 5A, the regulators will dissipate 17.35W; there are 20W heatsinks readily (and cheaply!) available for TO-3 casings, and this would be reasonable design.

The voltage output of the LM338 is 1.25V(1 + (R2/R1) + Iadj(R2). R1 is recommended to be 120 ohms; Iadj(max) (which is an error term) is 100uA, and Iadj(typ) is 45uA, but delta Iadj is stated as less than 5uA from 10mA to 5A output current, and from 3V to 35V dropped across the regulator; no minimum value is given, and since it is an error term we will set it to zero and 100uA to find our range. The output voltage is thus dependent upon R2. The ideal resistance of R2 for your 52V is therefore:

(1.25V(1 + (R2/120)) + 0uA(R2) = 52V
or
R2 = 120((52V/1.25V) - 1) = 4872 ohms
Refactoring 4.7k into the equation, we get 50.2V, a bit lower than your desired level; but if we go with 5.1k, the next higher 5% value, we get 54.375V, and we're into the dropout danger zone. At this level, 50.2V, you'll get 127W. The error term of 100uA x 120 ohms will give an additional 12mV, for a total of 50.212V; a miniscule increase.

On the capacitors, one thing you should be very aware of is that overloaded capacitors can explode. I have seen a 4700uF aluminum electrolytic cap explode with sufficient force to drive pieces of the aluminum case an inch or more deep into acoustic ceiling tiles ten feet above the bench, and that's at 12V, not 55V. When we made our first linear supplies in school, we were required to use a containment around the bench, and wear a face shield and welder's apron, before applying power to them for the first time. To this day I wear at least eye protection and most often a full transparent face shield when I first put power to a newly built supply. I do not recommend 63V caps in a 55V supply; should there be a power line surge, they would have no extra capacity to deal with it, and could be damaged and even explode. Bear in mind that there is no circuitry in your supply between the secondary of the transformer and the caps that would protect them. Supply surges happen all the time; you must design your supply so that it will tolerate them. Yet another reason to use a regulator; you won't have problems affording the caps that have sufficient voltage to handle surges. Let me put it this way: a 10000uF cap is probably equivalent to a respectable portion of a hand grenade. Think about it.

I would fuse the input to the transformer from the mains on the hot side, if you're running 230VAC (my guess since you seem to be from Britain) then at 2A, slo-blo of course, and fuse the output of the transformer to the rectifier bridges on the hot side at 5A, standard fuse; the LM339 has internal current-limiting circuitry that will protect the load from drawing more than 5A continuously. Make certain that you heatsink the rectifier bridges and the regulator extensively; consider a fan. If you really want to protect your amp, you can add a crowbar circuit to the output of the LM338, which will disconnect the 338 and short the DC line to the amp. I own a 10A 12V bench supply with a crowbar circuit, and can make a schematic available.

The inrush preventer is a good idea. It may preserve your fuses, and will reduce the stress on the entire system, transformer, rectifiers, caps, regulators, and amplifier, as well as your speakers. That giant thump you hear when you turn it on is not a Good Thing.

Safety ground is safety ground; I do not recommend putting anything but wire between the earth connection at the mains input to the facility (by "ground" or "earth," I mean the third wire that goes to the local hard earth ground where the mains enter your house- this is the safety ground- a green wire with a yellow stripe in the harmonized standard, and green in the old British standard) and the chassis; I also do not recommend tying the chassis to the neutral mains connection of course. If possible, a GFCI (you call them "RCDs-" "residual current devices" in Britain) would be a good idea. But remember: RCD/GFCI only protects against current that is lost to a local earth ground; phase-to-neutral (or phase-to-phase, if you happen to be using 3-phase) can still kill you and the RCD/GFCI won't do a dang thing about it, so don't get across a mains circuit. 230VAC can ruin your whole freakin' day, kill you very quickly, and even if it doesn't the burn scars will last a lifetime.

As a general practice, when designing a power supply, ensure that the components have at least 25% headroom for conditions that can cause any sort of problems if they go on for very long, 50% for conditions that will cause damage to the equipment if they go on for more than a second or so, and 100% for conditions that will cause immediate or instant failure of the equipment, or immediate or instant death or injury. The transformer can handle an overage for a little while before it becomes critical; but the rectifier diodes may be damaged and potentially cause damage elsewhere if they are exposed to something that's wrong for more than a second or so. Capacitors, to finish the lesson, can EXPLODE. Watch out.

 

[AndrewT]

Hi,
a good regulated power supply for the output stages of a power amp is just as complex a design job as the power amp itself. To cascade the reg and power amp will require some might debugging powers that will stretch any good designer.
Using a chip reg is a cop out that is unlikely to produce the goods.

I would (do) rely on brute force unregulated PSUs and they allow an amp to perform simply.

 

[jackinnj]

I would regulate the power supply

Unless the amplifier has known poor PSRR, regulation isn't necessarily a good idea. If the output devices fail, the regulator devices can be blown up, off the shelf regulators need to be heatsinked, when you regulate you put in another dimension of non-linearity into the equation, and you turn "music power" into wasted heat in the process.

wrt exploding capacitors -- check the ripple current rating on the manufacturer's datasheet -- this one went off like a shotgun shell:

An externally hosted image should be here but it was not working when we last tested it.

 

[Shredly]

Using a chip reg is a cop out that is unlikely to produce the goods.

I disagree- what is essential is that a good integrated regulator be used, and that it be adequately bypassed for stability.

Early regulator designs suffered from instability and had trouble dealing with ripple when used in adjustable configurations. Up until the mid- to late-'90s, they generally had current limits of an ampere or two. However, as competitive pressure from the switched-mode designs then becoming popular increased, the linear IC manufacturers saw their sales fall off, and decided they'd better get some designs capable of more. The LM339 was a result of this effort.

It handles 5A minimum continuous, typical is 8A, and 7 to 12A in half-millisecond pulses. It pushes ripple down by 75dB. It handles voltage input-to-output differentials of up to 35V, and the dropout is 3V or less in all supported temperature, current, and voltage regimes. Error voltage developed by bleed current from the adjust pin is minimized, at a maximum of 100uA. It is packaged in the venerable, reliable, well-supported and performant TO-3 package, capable of dissipating at 35C/W to the ambient without heatsinking, and with a very low thermal resistance of 1C/W to the case. (For those not familiar with thermal resistance calculations, with a maximum temperature of 125C, ambient at 20C, this is a difference of 105C, implying that the TO-3 case without heatsinking can dissipate 3W to the ambient. With a 1C/W case resistance, the device could dissipate 105W into an ideal ambient heatsink; with a well-cooled and properly thermally coupled real-world heatsink it should be capable of dissipating 50W easily, 80W with difficulty. Because there are a million different heatsinks for the TO-3, you can get about anything you can imagine.) The use of a minimal number of relatively small and inexpensive capacitors, two diodes, and two resistors, gives a highly serviceable, small, cheap, quiet, stable, mid-current, low- to mid-voltage fully regulated supply. The IC is internally protected from thermal overload, output short circuit, and the pass transistor is safe-area protected. If bypassed properly with protective diodes it is immune to almost anything that might happen in either the line or the load, with the exception of kilojoule-range power hits from lightning strikes and the like; and if you're running your amps without power protection, you're looking for trouble. I don't run anything more complicated than a can-opener without a power bar with thyristors in it.

With +/- 40V rails, 5A on each rail gives a maximum supply of 400W; rated conservatively, this will run a 200W RMS audio amplifier indefinitely. At 50V, you get 500W of supply and 250W RMS conservatively.

I actually intend to implement this design for my own use, and as a guitar player who plays regularly, you may be sure I will be pushing my equipment to the max on a regular basis. Any defects in this design will present themselves quite unmistakably, as I am very picky about my sound. I'll be using this supply with a Lin amp that I discussed elsewhere; the only reason it's not already implemented is because the toroids are on backorder at Digikey. I'm putting up with the power hum from the full-wave rectifier that's making it through the 6800uF caps, bypassed with .1uF ceramics, that Ampeg used in their brute-force supply design, and quite frankly it's driving me nuts, and always has. I'd never wish that hum on anyone. I just never got up the gumption to do anything about it; but since I've had to replace the output transistors, I figure the heck with it, it's already frankensteined, let's fix EVERYTHING.

I would (do) rely on brute force unregulated PSUs and they allow an amp to perform simply.

Well, I gotta ask- can you hear whether the amp's on or off without sending any signal through it? Have you measured the power hum? 6800uF is a heckuvalot of cap, and I got one on each rail, and I sure can hear it. Theoretically, that's 0.88V of ripple. About 2%. That means those two 6800uF caps are decreasing the ripple about 17dB or so- ridiculous for the price. 10,000uF would increase that... to a whopping 18dB. Gimme a 75dB regulator any day.

 

[Shredly]

Worth mentioning, while I was researching that last: http://sound.westhost.com/power-supplies.htm. Nice work.

 

[jackinnj]

LM338 max to ground potential is 40V.

 

[Shredly]

LM338 max to ground potential is 40V.

No, max from input to output. From the data sheet: "Since
the regulator is “floating” and sees only the input-to-output
differential voltage, supplies of several hundred volts can be
regulated as long as the maximum input to output differential
is not exceeded, i.e., do not short-circuit output to ground."

Perhaps it might be wise to add some protection on the output for a supply over 40V. It's a relatively simple circuit.

 

[richie00boy]

Well, I gotta ask- can you hear whether the amp's on or off without sending any signal through it? Have you measured the power hum? 6800uF is a heckuvalot of cap, and I got one on each rail, and I sure can hear it. Theoretically, that's 0.88V of ripple. About 2%. That means those two 6800uF caps are decreasing the ripple about 17dB or so- ridiculous for the price. 10,000uF would increase that... to a whopping 18dB. Gimme a 75dB regulator any day.

The unregulated caps may be a bit bigger, but nothing compared to the regulators heatsinks which are expensive, big and hot.

No, max from input to output. From the data sheet: "Since
the regulator is “floating” and sees only the input-to-output
differential voltage, supplies of several hundred volts can be
regulated as long as the maximum input to output differential
is not exceeded, i.e., do not short-circuit output to ground."

Which is exactly what will happen upon power-up when the output stability caps are discharged and present a very low impedance to ground.

 

[AndrewT]

Hi,
Shredly is entitled to his opinion.

Newcomers should stick to unregulated supplies until they have sufficient experience to advance.

The knowledgable will already have the skills to go regulated or not.

BTW Ipk for 140W into an 8ohm speaker is about 5.92Apk.
+-6m8F (puny certainly not brute force) of smoothing on each channel will sound pretty poor, particularly in the bass and/or sub-bass.
I would recommend a minimum of 2mF/Apk requiring +-12mF/channel & I have a leaning towards +-3mF/Apk/channel for bass duty. My overiding smoothing capacitor selection procedure is to adopt an RC time constant for the PSU ~=160mS to 200mS requiring +-20mF to +-25mF for 8ohm speakers and +-40mF to +-50mF for 4ohm speakers.

A well designed power amplifier will not become unstable nor hum with an appropriate quantity of ripple on the supply rails.

 

[jackinnj]

No, max from input to output. From the data sheet: "Since
the regulator is “floating” and sees only the input-to-output
differential voltage, supplies of several hundred volts can be
regulated as long as the maximum input to output differential
is not exceeded, i.e., do not short-circuit output to ground."

Perhaps it might be wise to add some protection on the output for a supply over 40V. It's a relatively simple circuit.

I think you have misread the data sheet. If you want the regulator to "float" it has to float on something -- a zener diode for instance. Here's a floating variant that I use in a tube amp:

An externally hosted image should be here but it was not working when we last tested it.

I did a group buy for a hundred boards of a variant which was used in the Jung and Hollander Marantz 7 and Last PAS modifications. Here's a writeup:

http://www.tech-diy.com/DIY_PWR_LastPass.htm

 

[Shredly]

Errrmmm, well, jackinnj, it does float on something: the LM338 requires both the 120 ohm resistor between output and adj, and a resistor from adj to ground, which adjusts the output voltage. See the data sheet's application hints section for more information. It's worth noting that the ST datasheet is considerably less informative than, and seems to have a few translation problems from, the National datasheet.

As far as the regulator heatsinks, richie00boy, if one is careful to adjust the output voltage as close as reasonable given line fluctuations to the regulator brownout voltage, then one finds that the dissipation in the pass transistor (and therefore by far the bulk of the dissipation in the regulator) at 5A is in the close neighborhood of 15-20W. Use of a fingered, on-board heatsink some 3" (80mm) tall, with a fan and good attention to airflow, is sufficient, and such heatsinks are available for $1 to $2 US each, including hardware and insulation pads. The footprint is the size of perhaps a 1mF cap, but the height is considerably less than a 3300uF.

It might be worth mentioning at this point that there is now an alternative to stinky, nasty, messy silicone heatsink grease; and I've now tested it in a 20A Cuk converter and my guitar amp's power amp's output stage and it works very, very nicely: it's a paraffin compound distributed by Digikey. It's a bit more expensive than the nasty silicone stuff, but it's non-toxic, non-smelly, doesn't outgas, and the application is as a stick of the material encased in a plastic holder; you just rub it onto the insulation pad and the heatsink, hit it with a heat gun for a minute, put some on the device's can's contact surface, and mount the device while the heatsink assembly is still hot but not too hot to handle. I was careful to not overstress the output transistors for the first hour or so of use, until they had been hot long enough that I felt good mechanical compliance was unquestionable, and I'd recommend this practice in general. It's one of the reasons manufacturers do burn-in before they sell a unit.

That said, richie00boy, thanks for pointing out the current outrush potential problem at startup; I'll look into this before I hook it up. I think that makes the current limit circuit at the output mandatory.

AndrewT, since there are two regulators each capable of delivering 5A continuous and a minimum of 7A for up to half a millisecond, with a typical value of 12A over the same period, I think that 5.92Apk should be supportable by the regulator. Note that the current limit on the regulator, bolstered by a limiting circuit at the output of the regulator circuit, should handle this smoothly, probably introducing clipping if the signal is beyond its load limits; the current limit of the device, according to the datasheet, smoothly decreases with time, rather than forcing hard shutdown. However, it is a valid point, and I'll look into it carefully, so thanks for bringing it up. I suspect this will limit my amp to below 180W total output; output amp clipping is EVIL where speakers are concerned.

In addition, AndrewT, it is apparent you have great experience with power amps; I have looked over some of your other posts and you seem to consistently have excellent advice. If I could reasonably ask it of you, I'd very much appreciate a little guidance on how I might address at least some of this hum problem on my power amp; the schematic is on another thread, so instead of spamming this one with a diagram that's off-topic, I'll provide a link to the thread. jaycee indicated that the problem might potentially be coming through the Zener diode used in the biasing circuit, but didn't indicate quite how. Any advice you can provide would be most helpful and very much appreciated.