EL84, Balanced Input, Push-Pull Amplifier

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Cressy Snr
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EL84, Balanced Input, Push-Pull Amplifier

Post by Cressy Snr » Mon Apr 08, 2013 9:32 pm

Here is my "new" EL84 push pull amplifier in action:

Image

It takes a balanced input from my Musical Fidelity M1 DAC, which has balanced outputs as an option on its back panel.

OK so it means you can't connect anything single ended to it, but never mind. The sound makes up for that relatively minor inconvenience.

I'll put up the cct shortly.

Only around 5W in triode mode and driven by 5687s, lolloping along at 7mA per side, but it's a start. It has rekindled my enthusiasm for a more powerful version.
Advantages are: no phase splitter, valve or otherwise.
Everything is kept strictly push-pull, from DAC rear panel, through to output transformer.

I can't really comment on the sound as I have changed too many things in one go, ie gone from pentode to triode mode,
losing 60% of my power, taken out the feedback loop, changed the drivers, but it's not half bad, though I say so myself.

I'll be bringing it to Owston, so if anyone having a DAC with balanced outputs, or a balanced preamp wants a go, they are welcome to have a play.
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Post by Cressy Snr » Mon Apr 08, 2013 10:00 pm

Here's the circuit with voltages and component values.

Image

As ever it is open to revision and open to advice as usual.
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Post by Cressy Snr » Mon Apr 08, 2013 11:02 pm

I reckon I could be losing out at the very top end with that 100K pot in there.
Input capacitance of the 5687 = 4pFa-g, mu = 18.

Trouble is I'm not quite sure how to work out the -3dB point of the resulting low pass filter as I'm not clear on what to take into account on the resistance side of the equation.

It is extremely easy to listen to, with a humungous cuddly, enveloping soundstage BTW :D

Instinct tells me to drop the pot value to 47K but I don't want to be forking out unnecessarily.

I could use a little guidance chaps.
Last edited by Cressy Snr on Mon Apr 08, 2013 11:14 pm, edited 1 time in total.
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Ali Tait
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Post by Ali Tait » Mon Apr 08, 2013 11:04 pm

My valve dac has balanced outs Steve, be interesting to try it out.

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Post by Cressy Snr » Mon Apr 08, 2013 11:13 pm

Ali Tait wrote:My valve dac has balanced outs Steve, be interesting to try it out.
Sounds like a plan to me Ali :)
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Post by Nick » Tue Apr 09, 2013 12:06 am

Why not try removing the resistors and pot, and using the DAC volume control to set the voll and see if there is any audible HF cut?
Resistance isn't futile it's V / I.

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Post by Cressy Snr » Tue Apr 09, 2013 10:26 am

Nick wrote:Why not try removing the resistors and pot, and using the DAC volume control to set the voll and see if there is any audible HF cut?
I was up late, listening last night. I think the loss of treble I was perceiving was down to a more neutral overall presentation.
Having not heard balanced equipment before as I am not used to pro gear, the presentation of treble detail was interesting. Things were just "there"
The most overarching characteristic of balanced operation seems to be"neat and tidy"; everything in its place.
"Character" is stripped away. It is quite easy to listen in to different parts of the recording. Then again I suppose that is precisely what a studio engineer would need. It is an honest, direct sound, free of artifice.

It has to be said that the sound of fully balanced, push-pull operation is a "different"sound. I think Nick, you will be pleasantly surprised if you go ahead with the balanced 300B push pull amplifier. I can imagine, this type of presentation would go down very well with fans of Quad ESLs :wink:

I like it, but then I like everything.
The most surprising thing for me is the fact that I wired the volume control right first time.
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Post by Mike H » Tue Apr 09, 2013 1:59 pm

I was going to say can't see how that pot arrangement there is different to 'normal' pot or shunted pot as far as treble goes (Miller cap etc.).
 
I have boxes and boxes full of components, but never the right ones!

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Post by Cressy Snr » Tue Apr 09, 2013 2:30 pm

Mike H wrote:I was going to say can't see how that pot arrangement there is different to 'normal' pot or shunted pot as far as treble goes (Miller cap etc.).
It was just something MJ mentioned, but as of now, I see no reason to change the pot and resistor values. :)

This balanced line lark, gives a whole new avenue to explore in terms of push-pull amplification.
No doubt our Ed has racks of the stuff in his studio, but for civilians like me it is a bit of an ear opener, if you know what I mean.
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Post by Cressy Snr » Tue Apr 09, 2013 2:33 pm

Should be fun trying Ali's balanced mains power supply with all this balanced gear. :)
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Post by Ali Tait » Tue Apr 09, 2013 3:49 pm

Aye, balanced from the plug to the speaker!

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Post by Nick » Tue Apr 09, 2013 7:30 pm

Well, I have built a few balanced things now. I think part of what you are hearing Steve is the lack of the treble damage your typical valve phase splitter produces.
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Post by Paul Barker » Tue Apr 09, 2013 7:51 pm

What were the phase splitter transformers you had nick on your EL34 which sounded great? Was it at the input?
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Post by Nick » Tue Apr 09, 2013 9:42 pm

They were SP Wound ones. Driven by a single ended 5687.

Part of the "will get to it" plan is a pair of SP Wound 1+1:1+1 interstages, I have it in mind to take the balanced input to a LTP of 5687 driving the primary, secondary to 300b grid, with harmonic equaliser (which that el34 amp had).
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Post by IslandPink » Tue Apr 09, 2013 9:45 pm

... and some excellent 6.6k A-A PP outputs with Litz-wire secondaries - available at a very reasonable 2nd-hand price :!:
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