Balanced interstages for PP

Design and use of the various types.
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Balanced interstages for PP

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Fellas

I'd like to introduce a friend, Jon Finlayson, to the group. Jon is a fellow Londoner who I've known for a few years now; he has a very nice ESL based system and was asking me about PP interstages for his 300B PP amps.

He has experimented with lundahl iron and a pair of custom Sowter units which he bought from me a few years ago. These are PP (1+1) to (2+2) and are on a half M6, half mumetal (80% nickel) core.

He wants to move these, which are preferred to the Lundahls, to his tweeter amp and so will need some new I/S iron for the 300B amps.

The 300Bs are driven pretty much full range by 6C45 in PP at 170V/12mA
each, so I reckon a (1+1) to (1+1) device should be fine. Having said that, a slight stepup might be beneficial if if didn't make the winding geometry a royal pain.

The dual rather than centre-tapped secondary is needed for fixed bias of the 300Bs. Bias is at about 65V, so peak signal swing is very roughly 100V rms across the full primary.

As for primary inductance: there is a touch of cathode degeneration on the 6C45s for the dc balancing, so I reckon 4k a-a impedance. The amps are run without any LP filtering at the moment, I was suggesting to Jon that he might try this as he is supplementing the ESLs with a MC bass system, and the ESLs aren't going to reproduce much below 50Hz. What do yo reckon Dave?

I should also point out that Jon, whilst too modest to make such a statement himself, has a tremendously acute set of ears; if there is any difference at all in sound between 2 components, he will spot it when I myself am asking if the music is even playing.

He also has a lot of iron in his system, what with TVCs and WOT preamps etc, so I think he should get at least some of Intact's stuff in there.

Jon - anything else to add?

Cheers
Shines
shinebox
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(that was me)

Post by shinebox »

moved from the AVC forum where I mistakenly posted before...
Sorry about that
finlayson
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transformer - interstage - P/P

Post by finlayson »

Thanks Shines for the intro. I guess I can manage well enough without any step up assuming that provides the ideal signal path. I'm stepping down 2:1 at the moment so 1:1 will seem loud!
dave slagle
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Post by dave slagle »

The 300Bs are driven pretty much full range by 6C45 in PP at 170V/12mA
How is he doing the splitting prior to that?
each, so I reckon a (1+1) to (1+1) device should be fine. Having said that, a slight stepup might be beneficial if if didn't make the winding geometry a royal pain.
Going 1+1:1+1 will allow for much better bandwidth so that would be my suggestion.
The dual rather than centre-tapped secondary is needed for fixed bias of the 300Bs.
No problem...
As for primary inductance: there is a touch of cathode degeneration on the 6C45s for the dc balancing, so I reckon 4k a-a impedance.
Lets talk inductance, DC imbalance and DCR here. I'd like to see a "fair" bit of inductance with say a butt gap, then depending on his ability to balance currents he could restack for up to 10X that number. I'd like to get away with 80% nickel but it appears as if that will make the DCR pretty high, plus 100V is a pretty hefty swing which makes a butt gapped 49% nickel core seem like the best choice.

I get something like 300-400 ohms primary DCR, 160hy's end to end, and saturation at 5ma of imbalance + 100V @ 20hz.

It would be very interesting to play around with small amounts of offset to see if there are any sonic changes with a DC bias.

The amps are run without any LP filtering at the moment, I was suggesting to Jon that he might try this as he is supplementing the ESLs with a MC bass system, and the ESLs aren't going to reproduce much below 50Hz. What do yo reckon Dave?
I reckon that increasing the gap should do a nice lowpass at 6dB per octave :-)

dave
finlayson
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Interstage/PP

Post by finlayson »

Hi Dave,
The phase splitting is carried out by the use of a nice S&B transformer in my Pre-amp. The "Eurydice" circuit. So all my amps have balanced inputs.

I'd like to keep any high pass filter quite low, say 40Hz or slightly higher?

I'll chat to Chris about the points I don't fully grasp yet!
Cheers,
Jon
shinebox
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Post by shinebox »

Thanks Dave; your thoughts were similar to mine - I've built some headroom into the figures, but I'm always bearing in mind that if we relax some of the requirements, we might be able to go to the 80% mumetal.

Jon is biasing the 300Bs at about -50V each, so worst case A1 swing is 70Vrms or 100V peak across the secondary. But I can't help but wonder if there is a bit of A2 going on in his setup. OTOH, he's most certainly not a headbanger...

Jon tells me he can balance to within 0.2mA on the 6C45, with drift of about 0.3mA, so let's say about 0.5mA.

Thus the choice to be made is:
a) A conservative design based around 49% nickel, 100Vrms at 20Hz

b) Can we push things to 70Vrms, 40Hz, maybe even 50Hz to reap the benefits of the 80% nickel stuff?

Do you think (b) is really pushing the DCR on the mumetal? If not, the choice is clear I think.

Cheers
shines
finlayson
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Interstage P/P

Post by finlayson »

Those of you who have heard the difference between 80% and 49% infer mouth watering implications. To anyone who has not had the aural experience, can you describe it in words?
dave slagle
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Post by dave slagle »

Hey guys,
Jon is biasing the 300Bs at about -50V each, so worst case A1 swing is 70Vrms or 100V peak across the secondary. But I can't help but wonder if there is a bit of A2 going on in his setup. OTOH, he's most certainly not a headbanger...
I have to wonder how much overdesign is really needed? Partridge suggested full power at 50hz was adequate for output stages so I don't see why this shouldn't hold true for voltage too.
Jon tells me he can balance to within 0.2mA on the 6C45, with drift of about 0.3mA, so let's say about 0.5mA.
Would a unbalance on purpose be out of the question?
a) A conservative design based around 49% nickel, 100Vrms at 20Hz
The true armchair engineers approach. Do note that the 49% nickel I have is a core size bigger so we actually get a double hit there. I do have a small amount of 49% the same size as the 80% and this would address any saturation issues.
b) Can we push things to 70Vrms, 40Hz, maybe even 50Hz to reap the benefits of the 80% nickel stuff?
Sure we can do that. Not to oversimplify this design, but the dominating factor is the needed AC swing at the lowest frequency of interest. Whatever you need to do that will give you plenty of inductance (even with a gap) So the tradeoff becomes DCR vs. Core size.
Do you think (b) is really pushing the DCR on the mumetal? If not, the choice is clear I think.
I dunno :-) it is a voltage gain situation so little or no current will flow which makes the DCR less of an issue to me. Your mention of the A2 peaks gets my attention a bit since things (in thoery) could get a bit odd in an A2 situation. To be honest, I still don't have my head around how to design for the occasional A2 grid with SE which makes PP A2 a blur to this armchair engineer :-)

My guess on the best way to proceed is to come up with a realistic signal level these transformers will see on average and spec that for 50hz and see where the DCR ends up. If we assume the 70V rms number at 50hz we could come in at 400 ohms DCR. Remember changing that 70V @ 50hz to 100V @ 20hz is almost a 4X increase in flux which requires 4X the turns resulting in a 16K increase in DCR for a given window.

Right now I am looking at 2900gauss for a 70V 50hz signal with the 80% nickel. With a butt gap I see 160hy and 1ma of offset will match the AC and DC flux levels just about saturation. Of course all of this is "Laptop" computing of armchair engineering but you have to start somewhere.

This also seems like the perfect situation to try swapping in a 49% nickel core to see where we stand, plus in the case that the 80% just doesn't cut it, the 49% will come to the rescue since the math suggests that the difference between the saturation of the 49% and 80% nickel is just about the difference between 100v @ 20hz and 70V @ 50hz.

If you guys are up for the listening and reporting back, I'd love to have your input on a number of different ideas that could be related to this situation.

dave
shinebox
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Post by shinebox »

Hey Dave,
Thanks for the detailed reply; to answer some of your points:

1) I'm sure Jon could dial in some deliberate DC imbalance to see what the effect would be.

2) I'm sure he'd be well up for trying out 49% vs 80% and reporting back. I would love to take part in that, but it will depend on whether I'll be living in London or Queens when it happens...

3) Rw the DCR- i'm not sure it's an issue. I was thinking more "high DCR" = high parasitics, but then with dual 1:1, not much of an issue I guess.

Will be interesting to compare the armchair vs artisanal approaches... if I'm making recommendations to others, I tend to err to the former, cos you just don't know how something is going to work with their listening tastes, setup, music etc. But part of me wonders if giving such advice is, well, erring.

Jon - as for the difference in 49% vs 80%, I have no experience of the 49% in direct comparison. I do know that the 80% can sound absolutely magnificent if not stressed, and if you have the opportunity to try both, on Dave's bobbins, well - how can you lose? I may even be able to escort the stuff across the pond for you...

cheers
finlayson
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P/P Interstage options

Post by finlayson »

Thanks Dave, for the suggestion, which allows me the best for my system and provides possible further experience for all of us. I could probably organise a small listening panel from our Audiocircle to see if our ears can detect fine differences. Thanks too for the suggestion (Shines) that these could be transported personally as test pieces. An obvious bonus if it comes off. What might the programme look like?
Regards,
Jon
dave slagle
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Post by dave slagle »

1) I'm sure Jon could dial in some deliberate DC imbalance to see what the effect would be.
cool!
2) I'm sure he'd be well up for trying out 49% vs 80% and reporting back. I would love to take part in that, but it will depend on whether I'll be living in London or Queens when it happens...
again, cool!

3) Rw the DCR- i'm not sure it's an issue. I was thinking more "high DCR" = high parasitics, but then with dual 1:1, not much of an issue I guess.
i think this is the case for the situation at hand. when the load becomes tough things may change (ie A2) but i'm looking into that use.

I just sprung for some 80% in the bigger core size so we could replicate the experiment with both cores on two sizes.

how do you guys want to proceed?

dave
finlayson
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balanced interstage tr.

Post by finlayson »

Thanks Dave for the response. How to proceed? Would you like to make a suggestion? I'll talk to Smiles too. Once the thinking is done I'm keen to proceed but I'm in your hands.
Jon
dave slagle
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Post by dave slagle »

i did a pair on the smaller 80% nickel cores and all looks good. I get about 36hy's per section with a butt gap (150hy end to end) the DCR's are 260 and 320 ohms per winding (i like using two different wire sizes) and everyting is fine even with 2ma of offset.

these should represent a good starting point. will you be using shinebox express for delivery?

dave
finlayson
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Interstages

Post by finlayson »

Good to hear these items are ready. Hopefully Shines will be listening and if a trip across the pond is imminent something can be arranged. I'll mail him.
shinebox
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Shinebox Express

Post by shinebox »

Allo
Nice timing - Shines just got to NYC...

I have a wedding over the memorial day weekend and will be flying back to blighty on the Tue or Wed (30 or 31 May).

Dave, which one of these best suits for a visit

1) I already have the 26th booked off, lunchtime/early afternoon visit?
Alternatively I could try and book a half or full day off work on

2) Thu 25th
3) Tue 30th

4) Some day between now and then if these are tricky for you but will have to check at work. Could really use a day off anyway

We can catch up and you can show me your BH measurement stuff too, which I promised I would have more of a think about.

Cheers
shines
finlayson
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P/P Interstage installed

Post by finlayson »

Hi Dave,
Shinebox delivered, I've installed no 1, and boy it too delivers!!
'Most impressed. Clarity and bandwidth. Early days - more listening to come after second is installed.
A query: We tested the original Lundahls and optimised a snubber to minimise resonance. The value was 8K2 resistor followed by 1200pf polystyrene cap. When I changed to Sowter interstages, Brian suggested I keep the network. I can't remeasure since its not my test equipment. In certain circumstances a non-optimised circuit may be better than none. What would be your guess in the new circumstances? Its not the sort of thing where you can do a quick A/B test!
I love the way you've made these items - Many thanks. Were there some alternative ways I should try them?
Best, Jon
dave slagle
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Post by dave slagle »

wowo that was quick... shines said he deliverd them yesterday :-)

if you have a snubber or any type of additional loading, dump it altogether and see how it sounds.

dave
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