Aikido line stage as an F4 voltage stage

TomS · « **on:** 9 Mar 2008, 08:40 pm »

I put together a couple of mono Aikido line stage boards from John Broskie to use as a voltage gain stage for the PassDIY F4 I just finished. I think it was actually a post somewhere from JoshK that put this idea in my head, so I researched it and came up with enough parts to build one this weekend (thanks Josh!).

This one is dual mono with dual 6CG7's for about +20dB of gain, just right for my bi-amped Emerald Physics setup using about -10dB cut to the Odyssey LF amps. I used an old Modwright tube stage power supply I had laying around (from a Sony 999ES Signature) to get regulated B+ of +285vdc and a regulated +12.6vdc heater supply (biased up 75v). I decided to put a 10k trimmer in series with R15, anticipating I'd have to tune the noise down, but this thing is absolutely dead quiet. I can't hear anything at all with my ear right up to the speaker.

The nice thing about this stage is it is very flexible, allowing a wide variety of tube types and gains without much fuss. With the right tubes and a couple changes it makes a fine headphone amp too. I don't even have all the right cathode resistors on hand and it seems to work without problem and sound very good. It's idling a little light, so once I get the last few PRP resistors from PartsConnexion I'll swap out the RatShack carbons I scrounged to get it running. It has a mix of Kiwame's and PRP's now, along with a 1uF Auricap as the output coupling cap (on the bottom side). As you can see from the pix, John's boards allow a wide variety of caps to be installed with the series of pads. It permits a combo of film and oil type caps to even be used in parallel. Very nice PCB's, heavy materials, and clearly silkscreened for parts to be installed either on top, bottom, or both.

Obviously, this is the ugly version to make sure it all works correctly and to evaluate it sonically. I haven't decided whether to stuff it in the F4 case or build up a separate case yet. I thought I'd listen to it for awhile before committing to the beauty project part, but it already sounds quite nice.

Below are a few pix, with the rest in my gallery
http://www.audiocircle.com/index.php?action=gallery;area=browse;album=1382

Tom

Gordy · « **Reply #1 on:** 9 Mar 2008, 10:22 pm »

Congratulations on the build Tom! An excellent choice... "but this thing is absolutely dead quiet. I can't hear anything at all with my ear right up to the speaker." Exactly as I found with my 12sn7 version, provided the 75v bias circuit is installed. I ran mine without it for a week or so, until Josh showed/sent me a phono amp schematic using the bias and I built my own. Dead quiet and I'm using $9.00 off the shelf tubes.

mgalusha · « **Reply #2 on:** 9 Mar 2008, 10:28 pm »

Very nice Tom. If it wasn't for the high voltages you could do the Altmann mounting to a chunk of spruce and show off those nice boards.

TomS · « **Reply #3 on:** 9 Mar 2008, 10:37 pm »

Quote from: mgalusha on 9 Mar 2008, 10:28 pm

Very nice Tom. If it wasn't for the high voltages you could do the Altmann mounting to a chunk of spruce and show off those nice boards.

Yeah, I need to get it in a box pretty quick or I'll have a cat with very straight hair

mgalusha · « **Reply #4 on:** 9 Mar 2008, 11:10 pm »

A very toasted cat with straight hair. Just kidding of course.

jrebman · « **Reply #5 on:** 10 Mar 2008, 01:08 am »

Tom,

Sounds very nice. Another very positive data point for the aikido. I'll be starting my 24v 6gm8 pre/headamp version very shortly, and hope it will sound as good as it's higher voltage brothers.

Then there's a pair of octal mono boards waiting to become either a preamp or headamp -- depenmding on how the 24 version pans out.

-- Jim

sts9fan · « **Reply #6 on:** 10 Mar 2008, 01:26 am »

The only thing is that the 24v does not have as much gain. Can it be pushed to 20db?

jrebman · « **Reply #7 on:** 10 Mar 2008, 01:47 am »

Sorry, should have explained that the 24v Aidido is not going to be used with an F4, and the 15 db gain is more than enough -- and I'll even be cutting that down to 9 with a pair of cinemag 2:1 output transformers.

I'm just always glad to hear of another person who likes their Aikido.

-- Jim

TomS · « **Reply #8 on:** 10 Mar 2008, 01:13 pm »

Quote from: Gordy on 9 Mar 2008, 10:22 pm

Congratulations on the build Tom! An excellent choice... "but this thing is absolutely dead quiet. I can't hear anything at all with my ear right up to the speaker." Exactly as I found with my 12sn7 version, provided the 75v bias circuit is installed. I ran mine without it for a week or so, until Josh showed/sent me a phono amp schematic using the bias and I built my own. Dead quiet and I'm using $9.00 off the shelf tubes.

For those using the Aikido as a line preamp with gain control (mine is currently fixed in this application), did you position the attenuator at the input or output side?

What type of attenuator have you used - pot, manual stepped, electronic stepped, etc?

Tom

JoshK · « **Reply #9 on:** 10 Mar 2008, 05:37 pm »

The pot (whatever you choose to use) comes before the Aikido stage. At least if you are following Broskie's suggestion. You could place a pot after I imagine but I don't have experience on how that would work out.

I think Gordy used a pot and is now switching over to remote controlled relay based stepped att. I was planning to use a stepped att (manual) I have lying around at first and maybe switch it later.

Gordy · « **Reply #10 on:** 10 Mar 2008, 08:07 pm »

Quote from: JoshK on 10 Mar 2008, 05:37 pm

I think Gordy used a pot and is now switching over to remote controlled relay based stepped att.

Exactly, an Alps pot and Goldpoint selector for now. I've the Dantimax kit about 95% complete, still need to build the ribbon cables and program it

http://electronics.dantimax.dk/Kits/index.html

poseidonsvoice · « **Reply #11 on:** 11 Mar 2008, 02:55 pm »

I think if you used a pot on the output, you would have a variable output impedance. This may or many not be okay depending on what you are driving after it. My plans are to use an attenuator at the input and to use the octal board for 6SN7's or 12SN7's aa. The other option I've been thinking about is to build Kevin Carters RAKK II with active output stage, but that has only 9 dB of gain. Might as well do both and have fun!

Must sound SAWEET! This is about as close to getting a 'hybrid' amp. Perfect gain structure as well, what more do you want than 20-25 dB?

Great work Tom!

Fellas, when you talk about the 75V bias, do you mean raising the filament heater voltage by 75V? If so, that's super easy. Its exactly what I do with my preamp I have now.

Anand.

JoshK · « **Reply #12 on:** 11 Mar 2008, 03:10 pm »

Quote from: poseidonsvoice on 11 Mar 2008, 02:55 pm

Fellas, when you talk about the 75V bias, do you mean raising the filament heater voltage by 75V? If so, that's super easy. Its exactly what I do with my preamp I have now.

Anand.

Yep, that is what they mean. It is just one of the many things that should be included in a tube preamp as good practice, like tightly twisting the heater wires. It takes stress off the cathode to heater insulation as well.

TomS · « **Reply #13 on:** 11 Mar 2008, 04:18 pm »

For heater bias, I just used a 100k/330k divider from 300v B+ with a bypass to get about 70v to the floated negative side of my 12.6vdc supply.

jrebman · « **Reply #14 on:** 11 Mar 2008, 06:27 pm »

Can one of you guys give a detailed written description of what this circuit looks like? Is this the same one Broski shows in his manuals?

Josh, twisting heater wires is indeed a good thing, but you can overdo this too -- 2 twists per inch is fine -- at least that's what the old-school engineers told me (and I won't mention anny names.)

-- Jim

TomS · « **Reply #15 on:** 11 Mar 2008, 06:48 pm »

Jim,

Mine is exactly like John Broskie's from his PCB manuals other than you still need a power supply, which is open to interpretation.

Tom

TomS · « **Reply #16 on:** 13 Mar 2008, 01:41 am »

Finally got a case around it with a nice maple top and satin black sides. Still waiting for connectors, but good enough to be put in service without any HV accidents. It measured out at right around +20db gain no sweat. A couple pix, others in gallery.

Tom

mgalusha · « **Reply #17 on:** 13 Mar 2008, 02:10 am »

Very nice Tom! Looks like you got your scope fixed as well.

TomS · « **Reply #18 on:** 13 Mar 2008, 02:13 am »

Quote from: mgalusha on 13 Mar 2008, 02:10 am

Very nice Tom! Looks like you got your scope fixed as well.

Actually, "eBay" delivers a new one right to your door!

JoshK · « **Reply #19 on:** 13 Mar 2008, 12:07 pm »

Quote from: JoshK on 11 Mar 2008, 03:10 pm

Quote from: poseidonsvoice on 11 Mar 2008, 02:55 pm
Fellas, when you talk about the 75V bias, do you mean raising the filament heater voltage by 75V? If so, that's super easy. Its exactly what I do with my preamp I have now.

Anand.

Yep, that is what they mean. It is just one of the many things that should be included in a tube preamp as good practice, like tightly twisting the heater wires. It takes stress off the cathode to heater insulation as well.

Actually I take that back. You don't always want to bias up the heaters. In this case you do because you are stacking the tubes on top of eachother and you want to take the stress off the cathode/heater insulation of the top triode. In a regular grounded cathode amp, I think it is more case by case. Best thing is to look at the datasheet to see what the max values are for the heater to cathode voltage and design around that. However, biasing up/down the heaters so that it is relatively near the cathode is usually a good thing for noise.