Valve preamp.

[Mark Hennessy]

Looks good, Tracy!

I note that the input impedance of this amp is rather low, at just 1k. Some valve circuits will struggle to drive that (as might some solid-state ones too). You could increase the input impedance by raising both the 1k and the 18k feedback resistor, but noise might become an issue.

If you're happy with op-amps, then an NE5534 will be happy to drive 1k. They are cheap and perform extremely well. The NE5532 is the dual version.

If making a valve pre-amp, that's outside my expertise, but it will definitely need a cathode follower at the output.

Thinking laterally, an audio step-down transformer would be a nice way to transform the impedance, but unless you have a few sitting around, the good ones are extremely expensive.

One more thing - looking at the Vbe multiplier, I note you have a preset connected between collector and base. This is risky. If that pre-set ever goes open-circuit, the transistor will turn off. This will lead to both the output transistors turning on, and that might be expensive! It's conventional to connect the pre-set between base and emitter (and include a series resistor that limits the current when the pre-set has been turned to zero ohms). If that's not easy to arrange with your construction, you could simply swap the NPN BC546 with a PNP transistor. Anything will do, as it's only seeing a couple of volts - the BC558 would be an obvious choice here (as I have hundreds of them!). Of course, the emitter and collector must be swapped around, but otherwise it should be easy. Oh, and I trust that transistor is on the heat sink

[Terry]

I note that the input impedance of this amp is rather low, at just 1k.

And that 100pF input capacitor is hardly likely to produce a broadband response!

[Diabolical Artificer]

Got you lads re oscillation. If you look at the pics grid stoppers are tight on the pins and checked the OP with a PC SA, several in fact inc Audacity, there's no HF oscillation just 2nd harmonic @ around 40dB, 3rd 10dB below that. THD is 0.5% @ 1khz/5.4v RMS OP, noise is 0.004v RMS. Frequency response could be better, but with this amp I wasn't striving for anything too mad, I wanted a bit of 2nd harmonic.

I was thinking of applying a bit of global NFB, but would that ruin the OP Z? AFAIK it should improve it.

Nice fettling Tracy, I like that style of HS/Q fixing, some nice capacitors too. I like using pins too for the IP's/OP's but use turret tags, it makes removing the board easier.

Andy.

[Mark Hennessy]

I note that the input impedance of this amp is rather low, at just 1k.

And that 100pF input capacitor is hardly likely to produce a broadband response!

I appreciate that's probably humour, as it's clearly a µ, and is drawn as a polarised cap. And you can see it on the photo of the completed boards.

Andy, let me know if you feel we should start a new thread for Tracy's amplifier

[Terry]

And that 100pF input capacitor is hardly likely to produce a broadband response!

I appreciate that's probably humour, as it's clearly a µ, and is drawn as a polarised cap. And you can see it on the photo of the completed boards.

No Mark, it was a very stupid mistake on my part.

At a quick glance I mistook the µ for a ρ. I also failed to spot the polarised symbol.

I looked at and admired the photo but still didn't spot my mistake!

   

[Diabolical Artificer]

"Andy, let me know if you feel we should start a new thread for Tracy's amplifier" No need thanks Mark.

Had some interesting issues today, not with the amp, but when testing. I'd noticed there was some hum when the amps and preamp were connected to my main speakers, so put one monoblock and the preamp on the bench and scoped the OP of the preamp and OP of the poweramp. I was wandering if there was some HF oscillation or other deliterious interaction. First I scoped the OP of the poweramp at 0 volume and at full, result less than 10mV of N. Preamp, pretty much the same. Tested pre and power amp on two different speakers, no noise. So just to make sure I scoped the OP of the pre and OP of the poweramp, what the flegnog! Power OP only 1v RMS and after turning off and on and re positioning a probe the protection on the poweramp tripped.

After some investigation i found one BNC socket on the dummy load is iffy, a sinewave OP taken off this was dancing about like it was being modulated by 50hz, power OP down and other odd faults. I've had this on a few occasions and ended up tearing DUT's apart looking for faults whilst using this dummy load, so it needs to come down off the shelf and taken apart and looked at.

Apart from those shinnanigans I hooked everything up and played a few records through the setup, it sounds grand, happy with the result.

Andy.

[BusyBee]

Thanks Andy. I was aware I had intruded a little. I take your point about turret tags. I use pins for almost all connections to this type of board.

I thought that you had mis-read my writing Terry. It can be poor at the best of times so I generally print most things. It was good though as I then noticed I had failed to notice that I had not labelled the resistor in the bottom of the bias circuit. This is 1kOhm.

Looks good, Tracy!

I note that the input impedance of this amp is rather low, at just 1k. Some valve circuits will struggle to drive that (as might some solid-state ones too). You could increase the input impedance by raising both the 1k and the 18k feedback resistor, but noise might become an issue.

If you're happy with op-amps, then an NE5534 will be happy to drive 1k. They are cheap and perform extremely well. The NE5532 is the dual version.

If making a valve pre-amp, that's outside my expertise, but it will definitely need a cathode follower at the output.

Thinking laterally, an audio step-down transformer would be a nice way to transform the impedance, but unless you have a few sitting around, the good ones are extremely expensive.

One more thing - looking at the Vbe multiplier, I note you have a preset connected between collector and base. This is risky. If that pre-set ever goes open-circuit, the transistor will turn off. This will lead to both the output transistors turning on, and that might be expensive! It's conventional to connect the pre-set between base and emitter (and include a series resistor that limits the current when the pre-set has been turned to zero ohms). If that's not easy to arrange with your construction, you could simply swap the NPN BC546 with a PNP transistor. Anything will do, as it's only seeing a couple of volts - the BC558 would be an obvious choice here (as I have hundreds of them!). Of course, the emitter and collector must be swapped around, but otherwise it should be easy. Oh, and I trust that transistor is on the heat sink

Thanks Mark!

Going through your views:

Bearing in mind that this amplifier design was a very big learning curve for me as the last, and only, push pull amplifier I have ever made was a more simple example from a kit quite a few years ago.

I would agree that the input impedance of the amp is low. Probably 5 or 10k would be better. When I determine what pre-amp I will make that decision. My original thoughs were to use the valve with emitter follower design that I tend to favour. This has a fairly low output impedance (if I remember right between 600 and 800 Ohms but is below 1k) so would work reasonably but there would be some loss. Your idea of an op-amp circuit is good as my main issue would be with tone control which may be easier with op-amp networks. A valve amp is another option. I would likely instigate the option for a plug in pre-amp. Here the low impedance may benefit if longish lines (being aware of possible issues driving by an emitter or cathode follower). I do have the odd small transistor matching transformer but I suspect their characteristics are not ideal.

With regard to the Vbe multiplier, I am glad you have mentioned this. I got very confused when I was designing things as one of the sites I spent some time on was Elliot Sound Products, which has helpful information. https://sound-au.com/amp_design.htm With regard to the circuit though their information, although now I have investigated and tested and know what they are getting at, is incorrect with a slider going OC. It originally threw me as I knew they were wrong. Your suggestion of the resistor going OC is much better and is probably what they really meant. Last night I put together an LTSpice simulation with the constant current drive and determined that, if the slider went OC, the voltage across the transistor would be just below 4V. Today I did some tests on one of the amps (carefully) and could momentarily turn the pot full up. This gave 4.4V across the transistor with about 3.3A supply current at 22.5V battery voltage so compares well with the simulation (the output transistors are rated at 10A but the heatsinks cannot handle the power for long and heated rapidly). Obviously an issue. For another option which I did notice on the website, I am thinking of protection diodes across the transistor, which would also help if the transistor failed too. I did vary the pot and take a few readings for reference. I will also think about the change of transistor as I do have some BC556's. I will also think more about the transistor on the heatsink as it is not at present, but the Elliot site also suggested that it should be if a Darlington output but not with a compound pair. All very confusing but doing some testing for limits is really helping. There is nothing like 'hands on'.

Thanks

Tracy

[Mark Hennessy]

Thanks Mark!

Going through your views:

Bearing in mind that this amplifier design was a very big learning curve for me as the last, and only, push pull amplifier I have ever made was a more simple example from a kit quite a few years ago.

Being honest, it's a pretty accomplished design for a first effort. Definitely something to be proud of. My feedback was intended to help guide you in your learning "journey", so I hope you found it to be constructive. The joy of audio is that there's always more to learn, but even quite basic circuits can give excellent results

I would agree that the input impedance of the amp is low. Probably 5 or 10k would be better. When I determine what pre-amp I will make that decision. My original thoughs were to use the valve with emitter follower design that I tend to favour. This has a fairly low output impedance (if I remember right between 600 and 800 Ohms but is below 1k) so would work reasonably but there would be some loss. Your idea of an op-amp circuit is good as my main issue would be with tone control which may be easier with op-amp networks. A valve amp is another option. I would likely instigate the option for a plug in pre-amp. Here the low impedance may benefit if longish lines (being aware of possible issues driving by an emitter or cathode follower). I do have the odd small transistor matching transformer but I suspect their characteristics are not ideal.

Emitter followers work well. In simple terms, the output impedance from an emitter follower depends on what is feeding it. If you take it from a valve circuit - say, a triode with a 100k anode load - then the output impedance will be around 1k. More (but not completely) precisely*, it will be 100k divided by the current gain (hfe) of the transistor. And that's in parallel with the resistor you put on the emitter.

*: As you probably know, hfe varies wildly from sample to sample, and with operating conditions, so "rules of thumb" are more than adequate for estimating this sort of stuff

Of course, there's nothing stopping you using a Darlington pair or a CPF to further reduce the output impedance.

You're right to consider the capacitance of long cables, but in practice it's usually not a big problem. Just to put some numbers on it, a good rule of thumb might be 100pF per metre. From a moderate source impedance like 1k, that gives a -3dB point of 160kHz for a 10 metre cable. Ideally, it's best to aim for something in the 50 to 100 ohm region, but that again is just a rule of thumb. Clearly, a 10k output impedance would be bad, as that's more like 16kHz (not that there's much up there in practice).

What might be more significant is loading effect.

With an output impedance of 1k feeding into an input impedance of 1k, you have a potential divider that is halving the voltage. Whether this matters depends on the situation, but obviously it's worth keeping in mind.

With regard to the Vbe multiplier, I am glad you have mentioned this. I got very confused when I was designing things as one of the sites I spent some time on was Elliot Sound Products, which has helpful information. https://sound-au.com/amp_design.htm With regard to the circuit though their information, although now I have investigated and tested and know what they are getting at, is incorrect with a slider going OC. It originally threw me as I knew they were wrong. Your suggestion of the resistor going OC is much better and is probably what they really meant. Last night I put together an LTSpice simulation with the constant current drive and determined that, if the slider went OC, the voltage across the transistor would be just below 4V. Today I did some tests on one of the amps (carefully) and could momentarily turn the pot full up. This gave 4.4V across the transistor with about 3.3A supply current at 22.5V battery voltage so compares well with the simulation (the output transistors are rated at 10A but the heatsinks cannot handle the power for long and heated rapidly). Obviously an issue. For another option which I did notice on the website, I am thinking of protection diodes across the transistor, which would also help if the transistor failed too. I did vary the pot and take a few readings for reference. I will also think about the change of transistor as I do have some BC556's.

If you've ever had a transistor amp blow up, or go into thermal runaway, then you will have been forced to think a lot about these innocent-looking Vbe multiplier

Putting the pre-set between base and emitter is done because we know that pre-sets can get noisy with age, and the worst-case scenario is crossover distortion (as opposed to fireworks when it's between the collector and base). Plenty of designers make that mistake though. If using a good quality pre-set, you'll probably be fine in practice. Rod's suggestion of putting a diode string across the Vbe multiplier is not something you see all that often - it could be considered cheap insurance, but get the Vbe multiplier right in the first place.

I will also think more about the transistor on the heatsink as it is not at present, but the Elliot site also suggested that it should be if a Darlington output but not with a compound pair. All very confusing but doing some testing for limits is really helping. There is nothing like 'hands on'.

Ah yes, as your output stage is CPF, the Vbe multiplier transistor needs to be in thermal contact with the drivers rather than the outputs.

[BusyBee]

Just a quick update on my amplifier.

I have gone back to the simulation of the Vbe multiplier and determined that, with a few adjustments it could be made safer. I have now changed the resistor between base / emitter to 820 Ohm and put a 1kOhm across the variable resistor. It was an easy option with the circuit as constructed. The net result is that the setup is far easier, and the maximum the quiescent supply current (essentially output transistor current when high) can be set to is just less than 100mA at max (essentially mimiking an OC slider). The result I found indicates that the value of the original variable is probably higher than optimum). My simulation results suggest that maximum current flow on an open circuit variable resistor may be handled too but I have not disconnected and tested this. An open circuit transistor would pose a problem which could be handled by protection diodes but my measurment of voltages across this with various settings indicated that it may be difficult to determine the ideal type and number of diodes as there is around a 0.3V window. This is partly due to my fairly small heatsinks limiting dissipation. From testing I would suggest (as many do) that the largest / most efficient heatsinks practical are the best bet for coping before protection circuitry comes in. As my transistors are well above required spec for this application (10A 100W) I will play close attention to fusing. My earlier findings suggest to me that, with the original adjustable resistor and maximum setting error (OC slider) there would be perhaps 10 seconds before transistor failure so time for a suitably low value fuse (working on 3.3A current flow as measured but obviously now not the same conditions). Obviously there is the condition of OC transistor but maybe unlikely as it is little stressed.

Looking at the results of a few tests I think the amplifiers need supply fusing at 1A, although I may try 800mA fuses at first, if I have them, as current when heavily overdriven seems to be around the 700mA region (beyond normal operation). Further checks when in operation.

This is probably as far as I am going with this at present as I need to organise a good stable power supply with one of the transformers I have. The current setup with batteries is a bit limited and I need a proper installation with good supply lines.

Thanks for your input Mark. You have cleared up a few things that were confusing me.

Tracy