Inexpensive balanced push-pull amp with DHT driver.

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JamesD
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#16

Post by JamesD »

Not that high anything over 8mA/V will do it and as for how it sounds - Gary Pimms Tabor gets rave reviews from people who otherwise like SE amps...

It also gets rave reviews from the PP crowd so it must do something right...

I guess we need someone to build one and bring it to Owston... maybe next time...

James
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#17

Post by IslandPink »

Yes, the Tabor is the best PP amp I've ever heard .
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Cressy Snr
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#18

Post by Cressy Snr »

OK so I'm chicken :)

Here is a far far simpler amp with just the 2 stages. I've grown of late to like simple things.

Image

I was afraid the thing would slew limit due to the parallel push pull output stage as I had no idea how to figure out the drive current needed.
I needed to find out how to calculate this slew current business, as it represented quite a big gap in my design ability.

It turned out that I was worrying needlessly.

Here are my calcs in case I did something wrong.

The output stage is biased at 12.5V, which means that the driver needs to swing 12.5Vpk to provide full power.
As configured, the driver will easily swing 30vpk with a balanced input from the DAC so no trouble there.

Using 6AQ5 in triode mode. The grid is biased at -12.5Vdc at idle. To develop full power output you want to create a peak input amplitude of 12.5Vpk, so that the grid just touches the 0V point.

To develop that max output at 20kHz you will need to move the grid at a rate of 2 * pi * 20,000 * 12.5V, or 1571000 volts per second, which equates to 1.57V/uS.

Input capacitance of a 6AQ5 is around 10pF, so times by mu+1 we get 10 * 10.5 = 105pF. Add wiring strays and we can round this to about 120pF

Multiply this by the fact that we have two parallel output valves per phase and we get C =240pF.
Each side of the DCC90 has to have sufficient current capability to drive the 240pF g1 node to 12.5Vpk @ 20kHz.

The current required to do this is expressed as I = C (uF) * Slew rate (V/uS)

Plugging in the numbers we got from our previous calculation.

I = 0.00024uF * 1.57V/uS = 0.0003768A

Which comes out at approx 0.4mA

Giving ourselves a 5 fold amount of headroom (Stephie Bench) to allow for HF programme peaks we arrive at a figure of about 2mA.

The stage idles at 3.7mA so we should be fine. (I think) :?
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Cressy Snr
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#19

Post by Cressy Snr »

OTOH
Now I understand why a 2A3 requiring 45V peak at 20KHz
is not going to be driven properly with an ECC83.

You just about get away with it, with both sides parallelled up, but then, all you end up doing is to transfer the problem to the input side, as the Miller capacitance of the parallelled ECC83 has now itself become crippling.

Far better to use a high Gm pentode such as C3m for 2 stages or a 5687 or such like, preceded by a voltage amplifier if you want 3 stages.
Obvious now innit!
My eyes have been opened :)
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JamesD
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#20

Post by JamesD »

Steve,

Small correction - the peak voltage swing isn't from 0V to Pk volts but from -pk volts to +pk volts so, in this case, twice 12.5V or 25 Volts swing into that capacitance.

Also keep in mind that this is the drive current that goes into the driven valve and one have to leave enough current in the driver for it to continue to operate linearly - for a triode that is probably four times the drive current (some say ten times). A pentode that has its screen grid voltage heavily stabilised can drive a higher percentage of its total current - probably up to 50% so only needs twice the drive maximum...

Its a really good job that full power at 20KHz is a rare requirement!!!!

J
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#21

Post by Cressy Snr »

JamesD wrote:Steve,

Small correction - the peak voltage swing isn't from 0V to Pk volts but from -pk volts to +pk volts so, in this case, twice 12.5V or 25 Volts swing into that capacitance.

Also keep in mind that this is the drive current that goes into the driven valve and one have to leave enough current in the driver for it to continue to operate linearly - for a triode that is probably four times the drive current (some say ten times). A pentode that has its screen grid voltage heavily stabilised can drive a higher percentage of its total current - probably up to 50% so only needs twice the drive maximum...

Its a really good job that full power at 20KHz is a rare requirement!!!! J
Thanks for the correction James. At least I got the formulae right.
Redoing the maths, I'm still reasonably OK. it's still worth trying. Hope I don't talk myself out of it.

Listening to the 13D1 into EL84 push pull amp, I'm having difficulty seeing how it could be bettered. It is just superb, embarrassingly so given the fugly looking chassis,
Last edited by Cressy Snr on Sun May 26, 2013 10:45 pm, edited 1 time in total.
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JamesD
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#22

Post by JamesD »

You got closer than I did with the calcs... I've used a spreadsheet I built years ago for slew rate and this week Nick pointed out that its wrong - I used the wrong gain expression and was out by about a factor of four... very embarrassing...

DO please make it as I'm really curious how it would compare with your EL84s...

J
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#23

Post by Cressy Snr »

Redoing the maths properly I get a 0.7 mA requirement to prevent slewing distortion at 20KHz. Times 5 for headroom, gives 3.5mA with 3.7mA idle current available.

The DCC90 will swing about 7mA peak at full chat with the balanced output from the DAC; not a disaster by any means ( Morgan Jones multiplies by 4 rather than 5, I've just discovered)

So the design is still viable. I'm hoping the AC heating will not be too noisy, given that we are using push-pull drive, with a centre-tapped cathode. This arrangement hopefully should reject most of the common mode filament hum, depending on how well balanced the sections are.

The 6AQ5s should be here on Wednesday, so it would be silly not to build the thing. :)
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IslandPink
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#24

Post by IslandPink »

It was interesting that the JamesD spreadsheet was difficult to justify technically :D but the results ( eg. 8ma to drive a 2A3 , 17ma to drive a 300B, 3ma for a 45 ) seemed to be spot-on in practice from our small circle of user results.
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#25

Post by Neal »

IslandPink wrote:It was interesting that the JamesD spreadsheet was difficult to justify technically :D but the results ( eg. 8ma to drive a 2A3 , 17ma to drive a 300B, 3ma for a 45 ) seemed to be spot-on in practice from our small circle of user results.
:D yep, I use 8ma to drive my 2A3 PP amp!
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#26

Post by Andrew »

SteveTheShadow wrote:
So the design is still viable. I'm hoping the AC heating will not be too noisy, given that we are using push-pull drive, with a centre-tapped cathode. This arrangement hopefully should reject most of the common mode filament hum, depending on how well balanced the sections are.
I have some DC heater boards left, happy to bring them to Owston, yours at cost.

Andrew
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Cressy Snr
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#27

Post by Cressy Snr »

Andrew wrote:
I have some DC heater boards left, happy to bring them to Owston, yours at cost.

Andrew
Sounds like a good idea :)
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#28

Post by Cressy Snr »

IslandPink wrote:It was interesting that the JamesD spreadsheet was difficult to justify technically :D but the results ( eg. 8ma to drive a 2A3 , 17ma to drive a 300B, 3ma for a 45 ) seemed to be spot-on in practice from our small circle of user results.
OOOOH you lot!

Here I am grappling with maths that do not come naturally to me, and all the time you had a spreadsheet hidden under your desks, laughing up your sleeves while I sweated with differential calculus and radians.

Shame on you :wink: :wink: :D
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IslandPink
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#29

Post by IslandPink »

Oops !
In my defence I only have it on the work PC as I got it from James via e-mail . I use it for essential calculations during tea breaks !
Nobody knows how it comes out with the right answers now, though - we are still thinking about it :?
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JamesD
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#30

Post by JamesD »

I'm in the process of correcting it - then I'll post it for everyone to tear it apart!

The mistake I made was to use simple pentode gain Rl*Gm+1 rather than mu+1 so it not correct for triodes but gives a good answer in practice!

Serendipity on top of stupidity or 'I'll take being lucky over being good'...

J
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