Zero Feedback Impedance Amplifiers

[traderbam]

Any questions?

Yes. Who wrote this review?

[Susan-Parker]

Originally posted by traderbam
Yes. Who wrote this review?

Written by K. Williams.

N.B. I have his permission to publish his review, as you will have noted he wrote it specifically in answer to a question on this thread.

Best wishes,
Susan.

[Susan-Parker]

Hi John,

Originally posted by jkeny
Susan, hope all is well with you.

Yes, thank you. My neck is just about 100% recovered.

I'm working my way to gathering parts for this amp - I currently have two toroidal trafos - Nuvotem 160VA 230V primary dual secondaries 2X25V which I'm hoping will be suitable as output trafos.

Can I use these as output inductors and ignore secondaries?

I am personally not so keen on using just a swinging inductor output, i.e. the speaker connected directly across the mosfet sources. However others have done this successfully.

My reasons are:

1. Distortion is higher for a given bias level and output wattage.
2. No protection for the loudspeaker voice coils if one of the mosfets lets out the magic smoke.
3. EMC/RFI issues (but that is my particular hobby-horse).

Although toroids have better flux efficiency and so can use a smaller core area per given power level they do saturate harder so I generally keep to the same design ratios as for the EI transformers. This is to specify them in watts at one fourth the 50/60 Hz VA rating, i.e. 160 VA is good for 40 Watts (nominal).

Also use a power supply voltage that is in DC no more than the AC rating of the output windings (in cases where you are using a swinging inductor setup). So with the above transformers you would only use a 24 volt DC supply.

Is STW34NB20 the best reasonably priced Mosfet to use and do you offer any matched pairs for sale?

Yes. I have tested a number of alternatives and although there are some slight improvements in some of the figures overall the STW34NB20 come out well ahead in overall performance, even without the advantage of the cost.

I can provide matched pairs if you are not able to source components locally. Please note however that I have to buy in whole tubes to select from and this necessitates my charging a higher than list price for these parts.

I will add these to my list of componets available (will try to do it this evening, otherwise some time tomorrow).

What trafo would you recommend for input use (preferably available from RS components)

1. At this time the Sowter is still the first choice.

2. I have a pair of Lundhal LL1680 on order to test as an alternative for those who can get these more readily than the Sowter part.

3. Otherwise something like a 15 or 30 VA 115+115 : 12+12 toroid

15 VA = RS Stock no: 257-4907
30 VA = RS Stock no: 257-4963

These will need to be driven by a headphone level output such as the TPA6120 based QRV-07 from Per-Anders...

... Or one of the Zeus pre-amp / line drivers.

Thanks for all your kind help and information in these threads
John

You are welcome.

Best wishes,
Susan.

[jkeny]

Hi Susan,

Thanks for the usually comprehensive reply.

I wasn't aware you had injured your neck - in which case hope you have a speedy recovery.

I will be running these using my precious Rogers LS3/5As and so don't want to risk these speakers - so I will take your advice and use the secondaries in the output trafos.

I believe 40 Watts will be ample for these speakers - not a headbanger at heart as is evidenced by my choice of speaker.

So something like 16-18V power supply trafos should be ok, I believe and from your website it would appear about 150VA would be ample? I will be salvaging power trafos from older equipment so I'm not sure about the VA ratings of these.

Has anybody tried SLA battery power (2x12V). I have about 8x5Ah batteries which I might try. What do you think - I know these will discharge fairly quickly but it will allow me to hear the amps before investing in more toroids.

I can buy STW34NB20 singly for €4.13 but I don't know how many I would need to purchase to find two matched pairs even if I could get them from the same batch?

I can provide matched pairs if you are not able to source components locally. Please note however that I have to buy in whole tubes to select from and this necessitates my charging a higher than list price for these parts.

Thanks, Susan, I don't wish to put you to any unecessary expense but I thought you might have some spare matched pairs from previous tests. If you feel that others may well be interested in matched pairs and you will be financially covered then I would be interested in two pairs.

Thanks Susan
John

[Susan-Parker]

Hi John,

Originally posted by jkeny
Hi Susan,
Thanks for the usually comprehensive reply.

My pleasure.

I wasn't aware you had injured your neck - in which case hope you have a speedy recovery.

I fell (foot in a rabbit hole whilst walking in some woods outside London) and whiplashed my neck badly. Taken a good four months to recover. Odd twinge or ache now and again, but otherwise okay, thanks.

I will be running these using my precious Rogers LS3/5As and so don't want to risk these speakers - so I will take your advice and use the secondaries in the output trafos.

Ah, very prudent. I also have a pair (well two actually) of LS3/5As and they are very good in tonal balance, but lousy in phase with an ordinary SS amp. However they do firm up and greatly improve with a Zeus amplifier, very noticeably.

I believe 40 Watts will be ample for these speakers - not a headbanger at heart as is evidenced by my choice of speaker.

40 watts should be ample.

So something like 16-18V power supply trafos should be ok, I believe and from your website it would appear about 150VA would be ample? I will be salvaging power trafos from older equipment so I'm not sure about the VA ratings of these.

If you are salvaging you should be looking for split primaries i.e. 120+120 (or 115+115) to get the push-pull, and two secondaries to be used in series that will give you at least 70 volts (relative to the mains input).

Please note that most ordinary EI mains transformers do NOT work as they don't have the coupling between the primaries and secondaries. If it has a dual chamber bobbin with the mains on one side and the outputs on the other then it isn't a starter.

Has anybody tried SLA battery power (2x12V). I have about 8x5Ah batteries which I might try. What do you think - I know these will discharge fairly quickly but it will allow me to hear the amps before investing in more toroids.

Zeus amps should run well from batteries.

I can buy STW34NB20 singly for €4.13 but I don't know how many I would need to purchase to find two matched pairs even if I could get them from the same batch?

The total amount of matches varies, on a good day one might get as many as four sets of pairs from 25 pieces (depends on how fussy one is being of course).

Thanks, Susan, I don't wish to put you to any unecessary expense but I thought you might have some spare matched pairs from previous tests. If you feel that others may well be interested in matched pairs and you will be financially covered then I would be interested in two pairs.

Okay, preliminary prices now up at:

http://www.audiophonics.com/audiophonics-parts.html

I am still working on the new website, so a lot of links are not yet functional. The basic ones along the top (except tech-info) should work at least to the first level.

Hope this helps.

Best wishes,
Susan.

[jkeny]

Susan,

Great work - love the new website design - clarifies & illustrates all the options. Hope the keyboard work isn't painful with your whiplash injury?

One thing you could add to parts section of the website is payment method.

Let me the first to order a set of six STW34NB20 from you - I will email you to get VAT & carriage details

If you are salvaging you should be looking for split primaries i.e. 120+120 (or 115+115) to get the push-pull, and two secondaries to be used in series that will give you at least 70 volts (relative to the mains input).

I'm not sure I understand you - I was just intending to salvage power supply trafos - can these not be standard EI trafos?

Its probably mute at the moment as I have SLAs(correction 7Ah not 5Ah) which I will use to start with

Regards
John

[Susan-Parker]

Hi John,

Sorry to be so slow in replying to your questions, I have had computer problems.

Originally posted by jkeny
Susan, Great work - love the new website design - clarifies & illustrates all the options. Hope the keyboard work isn't painful with your whiplash injury?

Thanks, still lots of work to do. Keyboard work is now okay.

One thing you could add to parts section of the website is payment method.

Let me the first to order a set of six STW34NB20 from you - I will email you to get VAT & carriage details

Not yet decided on how to do this as I had not really been thinking of myself as a parts vendor per se, just supplying some specific bits to those who want to have a go at building my amps.

I note that we should keep commercial aspects off these general discussion lists, so please always email me directly about any commercial or payment questions, thanks.

I'm not sure I understand you - I was just intending to salvage power supply trafos - can these not be standard EI trafos?

Its probably mute at the moment as I have SLAs(correction 7Ah not 5Ah) which I will use to start with

Regards
John

Sorry, my mistake. I was thinking you were talking about parts for output transformers not power supply transformers. Yes, standard EI should be fine.

Will be interested to hear how you get on with the batteries.

Best wishes,
Susan.

[jkeny]

Susan,

Sent you some emails but don't know if you are receiving due to your computer problems, so this is just a note to see that all is ok at your end?

Mods, apologies for the commercial nature of my last post - won't happen again - thanks Susan for pointing this out

Cheers
John

[padamiecki]

Dear Susan,
did you try your Zeuses with Aikido preamps (by John Broskie JRB from Tube Cad Journal)?

[Susan-Parker]

Dear padamiecki,

Thank you for your post.

Originally posted by padamiecki
Dear Susan, did you try your Zeuses with Aikido preamps (by John Broskie JRB from Tube Cad Journal)?

I don't think that that the Aikido preamp would be able to drive the 600 ohm load of a Zeus power amp stage with the 5687/6SN7/6AS7/etc. type tubes specified.

It would be fine if additional amplification was needed in front of a Zeus preamp-line driver (which is designed to work from modern hi-level 2Vac CD/DVD player outputs).

One could try with 6C33Cs of course, but I prefer transformer coupling rather than capacitor as the latter needs to be quite large to preserve the low end and I am less than happy with adding capacitors into the signal path (unless absolutely necessary). And yes, I might use something a bit smaller for the grounded-cathode stage, perhaps some 6C19Ns (as I just happen to have some of these!).

Best wishes,
Susan.