Graaf GM20 mods - help!

[dpaws]

Evening all - I'm afraid I'm a newbie...

I have a pair of Graafs GM20's (schematic via link) but the constant tweeking of bias and offset is driving me mad!

https://cid-a332bc3aefc89f6b.skydriv...0schematic.pdf

Is there a way of adding an auto bias circuit?

Also, can anyone suggest any tweeks / improvements? I've scoured the net but nothing forthcoming.

Ahh, can I reduce the gain / adapt the present circuit to suit a higher input voltage without using a pot and thus destroying the dynamics? I'm using a Weiss Medea DAC with +27dBu via XLR's....

Thanks guys

[Koifarm]

Maybe you can use this:

http://www.tentlabs.com/Products/Tub...24/page24.html

Ronny

[Luke]

Is there any room in these to mod? They seem to have done a great job packing it all in.

[dpaws]

Originally posted by Luke
Is there any room in these to mod? They seem to have done a great job packing it all in.

Well spotted - there's a little room and I can always extend the base downwards. (I'm also happy to construct a third box if necessary, or maybe two full width units, one for the power supplies, the other for the tubes etc)

I'm toying with the idea of a passive line level crossover using one amp for each channel but until I can get the amps running in a consistent and stable manner it's pointless...

BTW Thanks for the interest, TENT module noted, email sent.... Cheers!

[Luke]

these are very expensive and probably warrant going into the dealer before you start modding. Are they under warranty?

[dpaws]

Originally posted by Luke
these are very expensive and probably warrant going into the dealer before you start modding. Are they under warranty?

Not so expensive (~£2K), they've been serviced regularly but no warranty - it's either mod or sell... gulp! Fantastic liquid yet dynamic sound and huge transformers are the reason for hanging on...

[Koifarm]

There is a voltage divider at the input. R3/R5 and R2/R4 you can change value's to get other input voltage.

Ronny

[dpaws]

Originally posted by Koifarm
There is a voltage divider at the input. R3/R5 and R2/R4 you can change value's to get other input voltage.

Ronny

From Graaf....

Usually the gain of the preamplifiers is around 15 db, and yours is even 27 db, so it’s not easy. If you want to decrease the gain of GM 20, you can utilize an attenuator in parallel by input of GM 20, working on resistances R2, R3 and R4, R5. Doing so the dynamics will be very low, or disappears at all, this is important for you to know. You have to bring the R2, R3 to 32 KOhm instead of 24 KOhm, and R4,R5 to 16 KOhm instead of 24 KOhm.

This unfortunately kills the dynamics, I'm looking for a more subtle way, reducing further gain rather than "wasting" my pre-amp gain...

[Koifarm]

Oke, use a 6n6p tube in the first stage. It is fully compatible with the 6922 pin layout only the heater current is two times higher. But the Graaf amp has lots in reserve. Because of his lower amplication factor ( 22 i.s.o. 33 ) the input can be higher.

Another possibility is to use a Ecc82 ( 20 x ) tube but then you have to change heater pin layout. Or use noval adaptor.

Ronny

[dpaws]

Top idea, just the knowledgeable advice I need - many thanks Is there any sonic performance to chose between the two suggestions? Microphonics / a golden reputation? I've just searched google...

http://theothor.googlepages.com/6n6ptweakforgm-20

"After using the russian tube for a long time, the midbass hump became to annoying, so I couldn't live with it. After talking to a DIY buddy, I also concluded that using the 6n6p tube, the output impedance of the amplifier changed, and that's not so good."

Maybe the 6n6p is not a favourable option - do you agree with the impedance comment?

Seeing as you obviously understand a tube or two.... in your opinion will the Tent lab bias module work within the Graaf schematic?

Much obliged