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Otl

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OTL

Thanks for that - have attached link (I hope!) problem is I can't tell from this diagram which value to use - 22uf seems a little excessive (but may be correct) Someone who's built one might know what works perhaps?
 

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Hi,

2200uF is barely enough to keep hum on the output low enough in this design, the output stage is not completely symmetrical with regard to power supply rejection so you need quite good filtering to reduce hum to acceptable levels. In a later version Andrea used a CLC filter for the anode supply which is an idea.

In my OTL which has a similar output stage I use good quality elytics of totally 5500uF without chokes with very good result, the quality of the elytics especially ESR is more important than the cap value itself.

And ... and... are you shure this is not a fake?

I dont know if this was meant as a joke by Larry Lomax or not but no, it is not a joke, I know several that have built this amp successfully and although it can be improved the circuit work as it is presented.

Regards Hans
 
Thanks for the info. I should have said my intention is to use a CRC filer per rail using two 2200uf capacitors. Would you approve? I have checked the ESR and is around 0.05 Ohms.
This is my first attempt at OTL so any help/observations is most welcome.

David.
 
Bah. People cought using a resistor for filtering a power output stage should be shot. :D

It's class A anyway, which...is pretty pitiful, adding insult to injury as it's already a tube OTL to begin with. So I guess it doesn't really matter. Whatever.

Good rule of thumb: 2mF (2000uF) per ampere. A bit less at high voltage since more ripple can usually be tolerated (percentage-wise). As-is, if it needs as much filtering as claimed, I don't really have a problem with 4700uF, but 2 x 1000uF would be a lot better, particularly with 100mH between them.

Tim
 
Larry Lomax said:
It seems that power supply filter capacitors are a bummer high also! Is it? 4700uF charged at 150V are a bit explosive, are they?
And ... and... are you shure this is not a fake? Italians are used to play jokes any time! And when they are not, they exagerate a bit! :confused: :D


Bah.
Andrea Ciuffoli has many nice projects.
His Power Follower 99, for example, is a "must".
See comments in: http://www.diyaudio.com/forums/show...37&perpage=10&highlight=ciuffoli&pagenumber=1

Fake? Exagerations? No thanks. Only good ideas. :nod:
 
It's class A anyway

No it is not, the idle current is only 200mA so the output stage goes in AB1.

Good rule of thumb: 2mF (2000uF) per ampere
It would be OK for a symmetrical output stage e.g a SS amp where ripple from the power supply is balanced out but in this stage the balance is not so good as the PSRR is different in the 2 sides, looking from the power supply it is a cathode follower and a ordinary cathode grounded stage and they have quite different PSRR. This is one of the few disadvantages of serial connected OTL amps compared to a circlotron.

I should have said my intention is to use a CRC filer per rail using two 2200uf capacitors. Would you approve?

A CRC filter is not a good choice in this case as you want to have the lowest possible impedance and any resistor that has a noticeable effect on the hum will be to large.

Regards Hans
 
tubetvr said:

No it is not, the idle current is only 200mA so the output stage goes in AB1.

....Huh?

Wait a minute, then how the heck does the bottom get bias from a cathode resistor. Wow, that's crappy.

It would be OK for a symmetrical output stage e.g a SS amp where ripple from the power supply is balanced out but in this stage the balance is not so good as the PSRR is different in the 2 sides, looking from the power supply it is a cathode follower and a ordinary cathode grounded stage and they have quite different PSRR. This is one of the few disadvantages of serial connected OTL amps compared to a circlotron.

Note the cathodyne splitter is AC-grounded to the output, applying feedback, which combined with the crossed wire drive, makes them both appear as CFs.

At least that's what I recall, I've never sat down and looked at the circuit real hard.

It also has global NFB which will suppress hum to some extent.

A CRC filter is not a good choice in this case as you want to have the lowest possible impedance and any resistor that has a noticeable effect on the hum will be to large.

Like I said, anyone using a resistor to filter a power output stage should be punished severely. :D

(To be fair, low impedance is what the PSU caps are for. If it is indeed class AB, the DC impedance (whoa, that's a near oxymoron!) will certainly suffer.)

(Hmm, speaking of caps... you guys, yes you who despise any capacitor or transformer between tubes and the speaker: you realize the PSU caps and, in the above case, the cathode bypass cap, are as much in the signal path as any coupling cap, right?)

Tim
 
Wait a minute, then how the heck does the bottom get bias from a cathode resistor. Wow, that's crappy.

Agreed, it is crappy but it works, Andreas variant with fixed bias is obviously better.

Note the cathodyne splitter is AC-grounded to the output, applying feedback, which combined with the crossed wire drive, makes them both appear as CFs.

Yes, that is tru for the signal but if you look from the power supply there is still a CF and an ordinary cathode grounded stage with different PSRR. Ripple is reduced by feedback but it is still much higher than it would be from a completely balanced stage.

Regards Hans
 

PRR

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> Good rule of thumb: 2mF (2000uF) per ampere

I was taught 1,000uFd/Amp minimum, more is better. Same thing in the end.

> It would be OK for a symmetrical output stage

Right. 1,000-2,000uFd/Amp makes ripple "small" in that it will not put deep dips in your waveform at clipping, but may not be clean enough for a low-PSRR circuit. Sand-state and push-pull pentodes under NFB can be fed with that rule, many amps need less ripple. Getting really-low ripple in one stage is hard or impossible. So you start with ~1,000uFd/Amp to get "DC", then add L-C or R-C stages to get "very clean DC".

I can't be dogmatic about R-C filters in power amps. When capacitors were expensive, R-C sucked. But modern caps are so darn cheap, that a small R and monster C can work, and the cost/performance may be better than an L-C (chokes have some R and a lot of $).

>> the idle current is only 200mA so the output stage goes in AB1.

> then how the heck does the bottom get bias from a cathode resistor.

It can work for speech/music. If you try that with a full-power test-tone, sure it will mis-bias itself into distress. But near-clipped speech/music has average current far below peak current. If the cathode cap is big enough, it charges very little on each peak. If we are going only slightly into AB only on speech/music peaks, it may hardly budge.

> you realize the PSU caps and, in the above case, the cathode bypass cap, are as much in the signal path as any coupling cap, right?

No. I've never been able to convince any no-cap-head of that notion. Thevinin's Law? Current always comes from somewhere? Not one in ten electronic hackers understands it.
 
OTL

Have just completed the (first!) version of the A.C OTL - works fine - excellent sound marred only by some hum from output stages. Have decided to convert to seperate bias for each 6c33c.
Question for anyone with experience of OTL's - would it be sensible to set the correct bias (in this case to provide 200ma) to one half of the output pair. and then to adjust the other valves bias to produce 'no volts' across a dummy speaker load? This would appear to set the optimum bias point. Any thoughts please.
Regards,
David.
 
would it be sensible to set the correct bias (in this case to provide 200ma) to one half of the output pair. and then to adjust the other valves bias to produce 'no volts' across a dummy speaker load?

Yes, that is how I do it but I usually start by setting the bias voltage to the same value for both tubes. If you are using the floating ground type power supply that Ciuffoli used and do not adjust the bias voltages equal as a starting point there is a risk that the voltage over each tube will be very different especially at idle as you could have high negative bias and high anode voltage or low negative bias and low anode vcltage giving the same current and still 0V at the output.

Therefore I use this method:

1 adjust bias to same value, (a reasonable value that should give around 200mA)

2 adjust both bias voltages in parallell until bias is 200mA or what you want to have.

3 adjust bias on one tube for 0V on output

Regards Hans
 
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