Valve pre with variable feedback

[Lawrence001]

Ive taken a punt on this as it looks quite interesting. Seems to have 2x6SN7 and an EZ80 plus a magic eye which apparently shows you when the volume level is right (I was thinking could there be a way of measuring if the feedback level is optimal for the volume level or something).

Does anyone know if the variable feedback might work well for system matching, volume level, different enters of music/ quality of recordings etc. Are there any professionally made amps with this feature?


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[Lawrence001]

Would help if I attached the link

http://pages.ebay.com/link/?nav=item...d=272519961880



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[Arkless Electronics]

I believe from a look at that ebay listing that what it has is a switch to add positive feedback, rather than the normal negative, in order to cause it to distort a bit and be deliberately "over-blown" and more valvey than valvey than valveymcvalve.....
Variable negative feedback (rather than a switch to just turn feedback on or off) in a valve pre amp is perfectly possible though yes.

[Lawrence001]

Oh dear the feedback may not be quite what I wanted then. Set to zero it might make a nice cheap pre amp. He gave me the option to return anyway.


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[Lawrence001]

Although as long as it's "good" even order distortion you never know I might quite like It especially with my class D amp when some OTT valveness may not be unwanted.


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[Reffc]

Negative feedback is often added to a vale preamp to reduce gain, using resistors in the feedback loop. Something like a pre using 5687's for example will have a full 15dB of gain without feedback. Some designers prefer not to apply it to keep the circuit simpler and less uncluttered and in these cases you have to be careful about matching with a power amp. Many valve power amps of yesteryear have highish input sensitivity from perhaps 500mV upwards, when really, all that is needed today with most sources is around 1.5 to 2v for full output. That's when negative feedback helps with a valve preamp, without adding distortion. Positive feedback, as Jez indicates will add distortion and create a more euphonic sound. Why, in heaven's name anyone thinks this is a good idea for fidelity, and even markets something that adds distortion, is beyond me, but I guess for those wanting to relive the Helicon days of valve radiograms, I guess it'll do the trick and add that warm fuzzy glow! (ps...I rather like valve radiograms and would collect them if we had more room!).

[Lawrence001]

I'm hoping that this is not the case, the description is unclear and the seller clearly doesn't understand what was made for him. I'll report back at the weekend once I receive it and have a play,


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[RothwellAudio]

I agree, the seller's description suggests to me that he might not really know what he has. If nothing else, the case, transformers, valves etc. would make a nice project to modify and turn into something you like - that's assuming it's a tag board construction and easy to work on.
BTW, the idea of adding some distortion or "warmth" is ok if used wisely at the recording stage. For example, recording a lead vocal or a single instrument processed with something that adds a euphonic character is fine (imo) because it really doesn't do any harm. However, putting an entire mix through such a device might not be such a good idea because any intermodulation distortion tends to become objectionable when there is a collection of instruments/voices which you want to hear with good separation.

[Lawrence001]

Plus if it's not beneficial feedback I can set it to zero and have a single input pre amp. I want to try it with my class D power amp for some tubiness.


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[Lawrence001]

Well unfortunately one of the channels crackles, however I can hear some distortion on the working channel even at the lowest setting so it's definitely not a hifi pre amp. He's asked me to wait til the end of the month to send back as he's skint, which I said I'd do. Never had that happen on ebay before,


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