The Latest Push-Pull Build

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Cressy Snr
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#871 Re: The Latest Push-Pull Build

Post by Cressy Snr »

I believe there is something in this paper from Texas instruments about current feedback amplifiers that may get to the bottom of the reasons why my amp had limited bandwidth and why now the available bandwidth has increased. http://www.ti.com/lit/an/snoa364c/snoa364c.pdf

The maths in it is way beyond my understanding, the technology is different, but bode plots, crossover frequencies (nothing to do with speakers) poles and settling time are things I have seen in old books but (apart from poles) failed to get to grips with. Reading between the maths, the cathode follower, between the input stage and the output is helping a lot.

There is also something in there that explains the feedback resistor value (R2) doing things to the frequency response as well as altering the feedback levels. You have to be an engineer/mathematician to make full use of it, but I think there is something in there for poets. er ......I think
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Thermionic Idler
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#872 Re: The Latest Push-Pull Build

Post by Thermionic Idler »

Broskie touches on cathode followers here:

http://www.tubecad.com/2011/08/blog0212.htm
Deck: Garrard 301 - Audio Origami PU7 - NW Analogue DH3S
Phono: Pete Millett LR, Lundahl SUT, AMB σ22 PSU
Linestage: Bruno Putzeys Balanced Pre + Hans Polak relay mod, LCDuino control
Power amps: 300B PP 'Symmetric Reichert'
Speakers: FF225WK Big Mets
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Thermionic Idler
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#873 Re: The Latest Push-Pull Build

Post by Thermionic Idler »

Can I just confirm that my understanding is correct regarding the input stage.

I'm researching how one might configure this for monoblock power amp operation, specifically in a setup with the amps close to the speakers and long (probably circa 7m) balanced cables from a preamp with low output impedance, fully balanced.

I wondered if an input transformer might still be needed to help reject common mode noise before it gets to the rest of the circuit, but then I realised the input stage is differential anyway and should reject the majority of common mode noise before the cathode follower stage. Therefore even with long pre-power amp cables, the input transformer should not be needed (assuming careful grounding arrangements to prevent loops).

Have I got that right or should one still employ a transformer in that scenario?
Deck: Garrard 301 - Audio Origami PU7 - NW Analogue DH3S
Phono: Pete Millett LR, Lundahl SUT, AMB σ22 PSU
Linestage: Bruno Putzeys Balanced Pre + Hans Polak relay mod, LCDuino control
Power amps: 300B PP 'Symmetric Reichert'
Speakers: FF225WK Big Mets
Cressy Snr
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#874 Re: The Latest Push-Pull Build

Post by Cressy Snr »

If you are balancd end-to-end, then you should not need a transformer.
Keep us posted when you get going. :)
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Thermionic Idler
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#875 Re: The Latest Push-Pull Build

Post by Thermionic Idler »

That's good news.

Here's a very rough idea of the top panel layout that I sketched the other night - which should keep signal paths nice and short. I'm thinking side entry for XLR / speakers terminals on the right and mains lead / relay trigger on the left, so that they can sit close to the back wall on the floor (cage over the top).

And yes I know those are 5693's / 6SJ7's...

Image
Deck: Garrard 301 - Audio Origami PU7 - NW Analogue DH3S
Phono: Pete Millett LR, Lundahl SUT, AMB σ22 PSU
Linestage: Bruno Putzeys Balanced Pre + Hans Polak relay mod, LCDuino control
Power amps: 300B PP 'Symmetric Reichert'
Speakers: FF225WK Big Mets
Cressy Snr
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#876 Re: The Latest Push-Pull Build

Post by Cressy Snr »

Looks great Dave.
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Thermionic Idler
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#877 Re: The Latest Push-Pull Build

Post by Thermionic Idler »

Cheers. I've decided each chassis is going to be 17" x 10" - I had hoped to make them more compact but when I started predicting how much space the KT88's and decent output transformers and all that power supply gubbins would need… plus I already have two 17" x 10" valve cages from my old Transcendent T16's which I'll be able to re-use instead of having to buy new ones.

I do have to build the balanced preamp first… but planning is always fun.
Deck: Garrard 301 - Audio Origami PU7 - NW Analogue DH3S
Phono: Pete Millett LR, Lundahl SUT, AMB σ22 PSU
Linestage: Bruno Putzeys Balanced Pre + Hans Polak relay mod, LCDuino control
Power amps: 300B PP 'Symmetric Reichert'
Speakers: FF225WK Big Mets
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Thermionic Idler
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#878 Re: The Latest Push-Pull Build

Post by Thermionic Idler »

Here's a slightly better plan - after reading in MJ that's it's not good to have input valves within the flux field of the output transformer. I'll shut up about this now until something actually progresses beyond vapourware.

Image
Deck: Garrard 301 - Audio Origami PU7 - NW Analogue DH3S
Phono: Pete Millett LR, Lundahl SUT, AMB σ22 PSU
Linestage: Bruno Putzeys Balanced Pre + Hans Polak relay mod, LCDuino control
Power amps: 300B PP 'Symmetric Reichert'
Speakers: FF225WK Big Mets
Cressy Snr
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#879 Re: The Latest Push-Pull Build

Post by Cressy Snr »

Must admit Dave, that does look better.
You probably wouldn't have had a problem with the other layout, but then why take the chance that you might have.
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Cressy Snr
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#880 Re: The Latest Push-Pull Build

Post by Cressy Snr »

Yesterday and today I've been experimenting with getting as much gain out of the input pentodes as possible, including the use of starved current conditions. That's a nice advantage of the cathode follower buffer; you can play around with the pentode conditions within reason, with no penalty.

HT to the output stage has also been increased to 475V and the idle currents backed off to 30mA to compensate. Screen voltage to the KT88s has also been reduced from 230V to 200V. These adjustments have produced a more efficient overall gain structure, with no tonal penalties.

I found an earth fault on one of the left channel, shunt volume pot series resistors, which was shorting to the metal case of the volume control on the output to grid side, of one phase.
This would be a likely explanation as to why it was not possible (at Colin's) to get Nick's "Music First" phono stage to work correctly with my amp, when using its balanced outputs.

When I do the audio circuit rebuild, all the shunt pot components apart from the pot itself, will be mounted on a small piece of veroboard or similar, with short flying leads to the pot out of one end and the input cables to the other. This will get rid of the risk of earth faults.
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Thermionic Idler
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#881 Re: The Latest Push-Pull Build

Post by Thermionic Idler »

That's all good news - if I get around to building this myself I'd like to aim for about 40 watts per channel, a pair of KT88's in push pull ought to quite easily manage that I'd have thought? The Hashimoto output iron I'm looking at can take up to 60 watts.

Couple of questions Steve - reading through the thread it seems the power supply started off as a capacitor input CLC arrangement, but it now looks like a choke input power supply with two 2x 0.22uF 1kV "snubber" caps either side of the choke (similar to Morgan Jones page 318) - what are you using for those caps as I've been having trouble finding a cap that goes up to that voltage?

Also I assume the choke should be specced based on the power supply being 'choke input' rather than 'cap input' ? Sowter has them listed separately.

Thanks.

Edited to add my latest layout plan - I dusted off Turbocad as I figured I could plan wiring routes with it as well.

Image
Deck: Garrard 301 - Audio Origami PU7 - NW Analogue DH3S
Phono: Pete Millett LR, Lundahl SUT, AMB σ22 PSU
Linestage: Bruno Putzeys Balanced Pre + Hans Polak relay mod, LCDuino control
Power amps: 300B PP 'Symmetric Reichert'
Speakers: FF225WK Big Mets
Cressy Snr
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#882 Re: The Latest Push-Pull Build

Post by Cressy Snr »

Thermionic Idler wrote:
Couple of questions Steve - reading through the thread it seems the power supply started off as a capacitor input CLC arrangement, but it now looks like a choke input power supply with two 2x 0.22uF 1kV "snubber" caps either side of the choke (similar to Morgan Jones page 318) - what are you using for those caps as I've been having trouble finding a cap that goes up to that voltage?

Also I assume the choke should be specced based on the power supply being 'choke input' rather than 'cap input' ? Sowter has them listed separately.

Thanks.
Hi Dave

These are the red WIMA caps I am using as snubbers:
http://www.maplin.co.uk/p/wima-polyprop ... itor-n44cn a bit expensive from Maplin and I'm sure they could be had cheaper elsewhere.

Re the chokes,
I just use the Hammond 193M, 10H, 300mA chokes, available from Philip at Bluebell Audio and have had no trouble. Whether they are as good as the the "swinging" type chokes that used to be used on LC supplies I don't know, but they seem to work fine.
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Nick
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#883 Re: The Latest Push-Pull Build

Post by Nick »

SteveTheShadow wrote:
Re the chokes,
I just use the Hammond 193M, 10H, 300mA chokes, available from Philip at Bluebell Audio and have had no trouble. Whether they are as good as the the "swinging" type chokes that used to be used on LC supplies I don't know, but they seem to work fine.
Swinging chokes are different again. Sowter size their choke input chokes on the bases of them having the full AC HT voltage across them, where as for CLC use they only have the ripple voltage to deal with. MJ covers the differences.
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Cressy Snr
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#884 Re: The Latest Push-Pull Build

Post by Cressy Snr »

Thanks for the clarification Nick.
From memory MJ also says that anything under 15H for an LC supply will suffer from crippling AC current.
Whether my 10H Hammond choke is suffering, I don't know, but it doesn't buzz, or get anything other than warm.

The main reason I changed my supply to LC was the class B nature of the output stage. The LC gives a bit better regulation
under varying current than the CLC supply I originally started with.
Better still would be a high voltage mosfet regulator for the HT, which is the final piece of the Gizmo jigsaw, but I'd have to learn a bit more about noise, in order to make full use of one.
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#885 Re: The Latest Push-Pull Build

Post by Cressy Snr »

Thanks for the clarification Nick.
From memory MJ also says that anything under 15H for an LC supply will suffer from crippling AC current.
Whether my 10H Hammond choke is suffering, I don't know, but it doesn't buzz, or get anything other than warm.

The main reason I changed my supply to LC was the class B nature of the output stage. The LC gives a bit better regulation
under varying current than the CLC supply I originally started with.
Better still would be a high voltage mosfet regulator for the HT, which is the final piece of the Gizmo jigsaw, but I'd have to learn a bit more about quietening the thing down, in order to make full use of one.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
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