DIY Tube Preamp

[fdegrove]

Hi,

1)I think it would best if some of the other members of the Sydney area would help you out on this.
Basically with a B+ of ~ 300VDC you can go a long way tweaking to your hearts' content.
Add a decent choke,some caps in the PSU and your halfway there to audio bliss.

2)There are probably a few hundred of those on the net already.
I'm not going to defend or dismiss one circuit over the other but what is needed for a linestage is:

A good PSU.

A good sounding medium mu tube.(mu is amplication factor,medium is all in the range of say 10 to 40)

If you want to stick to the 12**7 range the following tubes come to mind:
12AU7 12AY7 12BH7 and I'm sure there are some others that would fit the bill.

Low enough output impedance to drive a reasonable length of interconnect.

Ciao,

[Joel]

Originally posted by FBLEAGH
1) instead of getting a custom xformer for this, is there a multi purpose tube transformer i can get ?? ie one i can use on later projects??

First off, I agree with all that frank just mentioned. As far as the transformer - decide on the circuit, then pick a suitable one. There is no need whatsoever to go ordering custom transformers for a simple preamp project!
Something in the range of an 80-100mA rating will work for nearly any preamp circuit. But a preamp is a Class A amp, and a 40mA unit feeding 2 6SN7's will get hot.

2) are there any other designs that are worth looking at ?? other topologies etc ?? I jsut want a nice sounding preamp, that i can tweak and modify later, but is a good starting point.

There are plenty out there. I would think a 12AY7 driving a paralleled 6SN7 in each channel, capacitively coupled to the output, would be an excellent place to start. Doubles as a crystal clean headphone amp too. I just built one myself, and use it daily. I'm working up a drawing of the schematic to post on here maybe tomorrow.

Joel

[Brett]

Originally posted by Joel
First off, I agree with all that frank just mentioned. As far as the transformer - decide on the circuit, then pick a suitable one. There is no need whatsoever to go ordering custom transformers for a simple preamp project!

Joel,
Please point to a source of off the shelf tube transformer manufacturers in Australia.

[miguel2]

Hi there,

It depends on the gain that you need, but for a CDP input I think a very simple pre is the best choice here.

I have an EF86 pre, one stage in triode mode. It is simple, the tube is screened (but a bit microphonic) and the EF86 was made for audio applications. The PSU I use is by far more complicated than the amp section. I tried an 12AU7 approach but prefer the EF86.

The problem here can arise with output impedance. This circuit likes it over 200k but most power amps are around 47k. I am thinking on how to solve this without another stage.

As for the transformer it is custom made. If you have some factory near then its quite simple. Mine was cheaper then an Hammond.

[Joel]

Originally posted by Brett
Please point to a source of off the shelf tube transformer manufacturers in Australia.

Brett, please point to one in new york city!! haha. Have you heard of the internet?
Seriously, I can't buy any parts locally - I get everything through the mail, and I would assume you could do the same - even in Australia.

G'day mate.

[fdegrove]

Here is a source:

LANCROFT

I can't see anything on powerxformers but they should be able to build those as well.

There must be others but this is as close to Sydney I could get.

Took me 10 seconds to find,so what's the big deal?

Ciao,

[Brett]

Originally posted by Joel
Brett, please point to one in new york city!! haha. Have you heard of the internet?
Seriously, I can't buy any parts locally - I get everything through the mail, and I would assume you could do the same - even in Australia.

Considering the number of people who live in NY and surrounds is more than we have in a country with a land area about the same as the continental US, I'm surprised.

There is no one down here who has a stock product line like Hammond, so you just can't ring up Parts Express and have one here the next day like I could easily if I were in NYC. Sure you can get one from overseas via mail but airmail from the US is expensive, and surface takes forever (3 months) and transformers arent lightweight. Plus most of the transformers out of the US have a 100-120V primary which is of little use here. Hammond do make some universal primary models, but they cost a lot more.

Frank is correct, that Bob at Lancroft could <b>make</b> one, as could Harbuch, Tortech, Torema, Black Art, Turner and about a half dozen others I could think of. But no one has a stock line that I'm aware of so it will have to be made. So it's a custom.

See how <b>There is no need whatsoever to go ordering custom transformers for a simple preamp project!</b> makes no sense in an Australian context.

[Joel]

Ok Brett, I stand corrected.
No need to get so bitter.

[miguel2]

My custom 120 VA / 6 voltage outputs xformer costed around 30 euros. I think the shipment of one from the USA to Portugal would cost around that!

[Joel]

Try this out. It's been working good for me.

If you want to furthur simplify it, it will work nearly the same with only a single 6SN7. You're just raising the output impedance. I didn't notice much, if any, change in sound or performance. I use it with 63 ohm Sony headphones, and it can drive them to pain. It's also my only preamp.
12AY7 can be replaced in the schematic by almost anything - AT7, AX7, 5755, 5751, etc. I found 1/2 of another 6SN7 just didn't have enough gain though. My current version of it uses a 6C8-G.
Power supply consists of a 300V-0-300V secondary, rated at 100mA. Two filters of 500ohm resistor/100MF cap + 500/40MF. 5Y3GT rectifier. Filament line is center-tapped and grounded.

Joel