Amplifier improvement advice please

[Nigel Goodwin]

another improvement would be to go further and replace the bootstrap with a constant current source....

It makes very little difference, the bootstrap itself is a constant current source for AC - as long as the capacitor is large enough for the lowest frequencies you're concerned with.

 

[jack121]

Hi. You reducded the output capacitor C2 from 4200uf to 220uf, why the need for such a large reduction.

Thanks.

 

[Nigel Goodwin]

Hi. You reducded the output capacitor C2 from 4200uf to 220uf, why the need for such a large reduction.

Well I didn't, and wouldn't have - I'd have used at least 470uF, and probably 1000uF or even 2200uF, it depends on speaker impedance and how low a frequency you want to reach down to. However, 220uF may be perfectly fine - no point in feeding 20Hz bass to a 2 inch speaker in a little radio.

220uF gives about 90Hz at 3dB down with an 8 ohm speaker, or 180Hz for a 4 ohm speaker.

 

[rjenkinsgb]

Hi. You reducded the output capacitor C2 from 4200uf to 220uf, why the need for such a large reduction.

It should be 4700uF, I must have copied & pasted the cap to that position and forgot to change the value

 

[Nigel Goodwin]

It should be 4700uF, I must have copied & pasted the cap to that position and forgot to change the value

Bit of overkill anyway - do you need 5Hz?. 1000uF would do nicely for 20Hz at 8 ohms.

 

[rjenkinsgb]

It was supposed to be incremental changes from the original design, nothing not beneficial.
That's what the original had.

If I wanted near flat response to 20Hz, I'd want one somewhat bigger still; 1000uF has a reactance of almost exactly eight ohms at 20Hz, so would be significantly attenuating the output power.

 

[Nigel Goodwin]

It was supposed to be incremental changes from the original design, nothing not beneficial.
That's what the original had.

If I wanted near flat response to 20Hz, I'd want one somewhat bigger still; 1000uF has a reactance of almost exactly eight ohms at 20Hz, so would be significantly attenuating the output power.

3dB, which is what amplifiers are (or were) commonly specified as at 20Hz. I was quite amused when I noticed how convenient 1000uF was

 

[audioguru]

Since the sensitivity of our hearing is logarithmic then -3dB (half the power) is only a small reduction in loudness. 1/10th sounds half as loud.

 

[unclejed613]

back in the day... the Dynaco SCA-80 amp had 10,000uf output caps, and it was a fairly successful amplifier for it's time.

 

[Nigel Goodwin]

back in the day... the Dynaco SCA-80 amp had 10,000uf output caps, and it was a fairly successful amplifier for it's time.

As discussed throughout this thread, the larger the capacitor the lower the bass response goes - and 10,000uF takes it down to 4Hz at 4 ohms. I doubt you could tell any difference with a smaller one though?, presumably it was all about bragging rights from the figures?. However, checking the schematic shows 5000uF, for both output and reservoir, it's also quite a peculiar design, presumably old enough to be before they knew what they were doing

 

[jack121]

Just i saw the input cap is 220uf, the bootstrapping cap you recommended is 220uf and the output cap is 220uf, thought is that conincidential or is there a something more here.

 

[Nigel Goodwin]

Just i saw the input cap is 220uf, the bootstrapping cap you recommended is 220uf and the output cap is 220uf, thought is that conincidential or is there a something more here.

Perhaps you should read post #24?.

 

[audioguru]

The Hackaday low power amplifier circuit was designed completely wrong with its extremely high voltage and 15A transistors and diodes upside down.
Nobody calculated that its 220uF input capacitor passes earthquake frequencies down to 0.36Hz and instead a 2.2uF capacitor would pass deep bass audio frequencies.

 

[audioguru]

Here is my hifi bootstrapped circuit:

 

[jack121]

Thanks!

I am using this amp with an active subwoofer. The sub handles all frequencies below 100hz with an attenutaion rate of 24db/octave.
This hackaday amp we are considering here handles everything above 100hz.
What would your input capacitor size be for this crossover frequency.
Also would it hurt to increase the size of the bootstrap capacitor to provide more current, or is 220-330uf enough.

Many thanks.

 

[audioguru]

The input capacitor by itself produces a very gradual low frequency cut of only 6dB per octave. Its -3dB cutoff frequency is 100Hz when it is 680nF.
The bootstrap capacitor works perfectly down to 5Hz so its value does not need to be increased.

I almost asked you why do you need a puny 2W hifi system when I thought of my hifi modifications to my 1/2W clock radio. It sounds great.

 

[unclejed613]

However, checking the schematic shows 5000uF, for both output and reservoir,

aftermarket replacements for these seem to all be 10,000uf, but they are "universal" replacements intended for the whole series of Dynaco amps...

 

[Nigel Goodwin]

aftermarket replacements for these seem to all be 10,000uf, but they are "universal" replacements intended for the whole series of Dynaco amps...

The classic Quad 303 used 2000uF - and unfortunately the capacitors were mounted above the boards, and due to the units very extended life the capacitors tended to leak all over one of the PCB's. Despite them been decades old, back when Quad was still run by the Acoustic Manufacturing Co. they produced a mod kit for them. Basically you fitted the chassis mount capacitors the other way up, and ran wires to where they needed to go

 

[jack121]

Do you think it may be possible to coax out a few more watts from this amp by raising the supply voltage. At the moment it is 12 volts, i do have a 18 volt supply and a 25 volt, what could i safely go up to.

At the moment i have only applied the first mod as shown in post #14 by rjenkingsgb

Thank you.

 

[Nigel Goodwin]

Do you think it may be possible to coax out a few more watts from this amp by raising the supply voltage. At the moment it is 12 volts, i do have a 18 volt supply and a 25 volt, what could i safely go up to.

I suspect everyone, I certainly did, assumed this was just a theoretical school project - and not something you were thinking of building, even with all the mods it's still a fairly naff poor amplifier, not something I would ever consider using.

Particularly for use with a sub-woofer - I would suggest a proper amplifier design, using a decent supply voltage so you get decent power out. Or use one of the many IC designs available.

You can even buy chinese kits using TDA2020's etc. for VERY little money.