Feedback fun with K5881 - Re-visited!

[Richard]

There's still a lot of interest in K5881 which was one of WAD's earlier designs from Oct 1993. The amp was based around the Mullard 5-20 circuit and later changed to the MKII version using a 7025 in SRPP instead of the EF86.

Later AG designs for WAD; Kel34, Kit88, and Kel80, were also based around the 5-20 but developed further to use a clever arrangement for applying global feedback.

This can be done with K5881 as explained below by NealG back in 2006. The original thread now has some corrupted text and no drawings but Neal's post is copied below with a new drawing added for anyone wanting to try it. The original thread is here for anyone wanting a read,

http://www.world-designs.co.uk/forum...read.php?t=410

Originally Posted by NealG

Following on from this thread, I thought I would try AG’s cct on my own 2a3 PP and also K5881MKII to see what effect it had. AG’s cct is clever and designed to work around LF instability by adding more feed back at LF. The amp actually becomes more stable due to LF phase correction, bass is improved as a result. I have to say I agree with this, on both amps I've tried Bass is noticeably improved, as is the midrange for some reason. Both amps seem more tuneful and ‘in control’. K5881 still sounds a little closed in to my ears as do most valve amps with high feedback but the mod has managed to bring out more controlled Bass and midrange detail.

This following should be applicable to any amp which implements loop feedback in the same manner as the K5881, which is based on the Mullard 5-20 cct.

Here’s what you need to do using K5881 MK2 as a reference.

Remove R3, Remove R4, Remove C2, Remove R18

Put R3 in the place of R18.

Put a 1000uf 10v or 16v quality ‘lytic cap in place of R4.

Add a 11R resistor in place of R3 (two 22R in parallel)

Move the feedback wire from the junction of R3/R4 to the junction of R3 and Cathode of V1




For K5881 MK1 the procedure is the same except that R3 becomes 33R not 11R as with K5881 MKII.

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Editted in after following posts in this thread,

<< Also it will be necessary to adjust C10 for best Sqr Wave performance. Change from 220pF to 1.2 ~ 1.5nF. This is best performed wile under test with a 'scope, however, if this cannot be done use a 1.5nF polystyrene cap. For K5881 MK1 use 1.5nF ~ 2nF, this has not been tested. >>

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If you’re thinking of using this in another similar cct work out the PD ratio formed by the existing f/b resistor and the shunt resistor (that is R4 divided by R4 plus R18). When you move the BIAS resistor (R3) to the f/b resistor position you need to recalculate the shunt to get the same ratio as before. If you don’t your f/b level will be different. I aimed for a 10Hz rolloff with the cap value. It can be tweaked a little in value to change Bass response.