13-12-2019, 04:42 PM
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.
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
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.
(11-12-2019, 08:41 PM)Mark Hennessy Wrote: 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
