Finally, I've tried S&B TX102 TVC with a GC amp

[carlosfm]

Evolution.

As you see Peter, ultra minimalist may not always be the way to go.
I'm very happy with the sound of my pre (OPA627+BUF634) working with my GC power amp (LM3875).

It's the best combination I've tried so far, and the first one I made.
And I tried several chips and implementations.

Some people reported very good results with an op-amp as a buffer, or even a valve.
I tried op-amp as a buffer and liked the results.

You now tried transformers and say it's very good.

This is evolution.

Deam it, your boxes are s'beautiful.
Come around, I need help with the boxes.

 

[Tomek]

Hello, I would like try transformers within passive "preamp", but the one mentioned here are out of the league (pricewise). I have found in my junk bin two mini transformers made by Tamura ( MIT-17). I think, those are impedance matching ones. Primary is 15kOhms, secondary is 600Ohms with CT. The only problem is their bandwith: 300Hz - 100kHz. Can those transformers be used in passive "preamp"?
Thanks

 

[Brett]

Hello, I would like try transformers within passive "preamp", but the one mentioned here are out of the league (pricewise). I have found in my junk bin two mini transformers made by Tamura ( MIT-17). I think, those are impedance matching ones. Primary is 15kOhms, secondary is 600Ohms with CT. The only problem is their bandwith: 300Hz - 100kHz. Can those transformers be used in passive "preamp"?
Thanks

Remember they'll have a loss of 5x. You might be able to improve the bottom end by driving them with a low source impedance. If you have them try them anyway, as their spec might be for full power, and at lower levels they have a better response.

 

[randytsuch]

Hello, I would like try transformers within passive "preamp", but the one mentioned here are out of the league (pricewise). I have found in my junk bin two mini transformers made by Tamura ( MIT-17). I think, those are impedance matching ones. Primary is 15kOhms, secondary is 600Ohms with CT. The only problem is their bandwith: 300Hz - 100kHz. Can those transformers be used in passive "preamp"?
Thanks

A more basic issue is they have to be made to be a TVC. A transformer for a TVC should have maybe 20 or more taps. Each tap ends up going to a switch. Selecting a different tap gives you a different volume. Since you talk about CT, it does not sound like the right kind of transformer.

BTW, I think TVC transformers are expensive because they are probably a lot of work, making all those taps.

Companies that make the right transformers, that I know of are S&B, sold through Bent audio over here, Audio Consulting, sold through referenceaudiomods and sowther (sp?), who does not have a US distributor that I know of.

I once saw a review when someone compared different TVC's, I will see if I can figure out where I saw it.

Randy

 

[Brett]

A more basic issue is they have to be made to be a TVC. A transformer for a TVC should have maybe 20 or more taps. Each tap ends up going to a switch. Selecting a different tap gives you a different volume. Since you talk about CT, it does not sound like the right kind of transformer.

They're definitely not TVC's.

 

[Peter Daniel]

I once saw a review when someone compared different TVC's, I will see if I can figure out where I saw it.

One such review is here: http://groups.yahoo.com/group/doctorjohn_CheapTubeAudio/message/3054

 

[randytsuch]

Hi Peter,
Thanks, that was the one I remembered. You saved me from look around for it.

Randy

 

[JCoffey]

I've tried searching around for more info on this idea but havent really been able to come up with much. Are there schematics for these preamps? Is it just a step att. connected to all the different leads of the transformers?

 

[zygibajt]

Yes it is.

 

[randytsuch]

I've tried searching around for more info on this idea but havent really been able to come up with much. Are there schematics for these preamps? Is it just a step att. connected to all the different leads of the transformers?

Go here
http://www.bentaudio.com/index2.html
and click on downloads.

Randy

 

[Brett]

Is it just a step att. connected to all the different leads of the transformers?

No. The transformer has a number of different windings and taps that are then connected to a switch. The turns ratio of the taps means that when different taps are selected via the switch there's a known amount of attenuation.

The schematics Randy linked to should make it clear.

 

[JCoffey]

Thats pretty much what I figured after looking at the schematics. But,where you you get the switch? Does it come with the transformers or is there a special one that I'd need?

 

[Brett]

Thats pretty much what I figured after looking at the schematics. But,where you you get the switch? Does it come with the transformers or is there a special one that I'd need?

The switches usually sell seperately, but I beleive Bent offers some packages with them included. If rolling your own, any 24way 2 pole quality switch will do; Elma, Seiden, Shallco etc

 

[randytsuch]

The switches usually sell seperately, but I beleive Bent offers some packages with them included. If rolling your own, any 24way 2 pole quality switch will do; Elma, Seiden, Shallco etc

Bent sells the Seiden switches, which look really nice, but are not cheap. I was going kind of low budget, and used electroswitches. If I every redo my preamp all out, I would probably use the Seidens.

I actually used a 3 pole switch, and used the middle switch as a shield, with grounded copper tape all around the middle switch. The Seidens are shielded already, so you don't need the DIY shield if you use those.

Randy

 

[Peter Daniel]

Of course the current version has still some rough edges...

I addressed most of the weak spots of previous design and here how the improved preamp looks. This is by no means a commercial version, but something I built yesterday for myself.

Three layers of acrylic are used here and no more gold plated hardware, but polished brass: looks much better this way. I also added 0.5" to the width and this improved final appearance.

 

[fedde]

Nice!!! Too bad that it just have been christmas, would have been a nice present for me under the tree

Fedde

 

[carlosfm]

Send me that for a review, Peter.
I may not send it back.

 

[elizard]

Lets see the back!

Wow btw, that looks AMAZING!!! I like the addition of the 2 layers of acrylic, kind of completes the look
Send one my way

Or you can take a trip to here in the summer (nobody wants to come here in winter for a good reason).

 

[Peter Daniel]

I used Cardas RCAs this time, as well as short toggle switches.

 

[elizard]

That's too much. I have to say that it looks better than 99.9% of any diy projects, or any commercial products. Actually, I can't think of something that really looks this good. A work of art I say!.

So your wife is tolerant of all this? Where do I find one of these? (j/k my gf is actually VERY tolerant of my hobbies, so long as 100% of the money and time doesn't go towards them - i.e. so long as i don't ignore her .. she even attentively listens to me ramble on about diy for a long while before starting to get a little bored! a real keeper!)