Question about various topologies for tube preamp

[Tubetec]

I suppose you could try that ,but my guess is that the amount of gain range you would get wouldnt be massive.
just found this ,
7db difference with 12ax7 bypassed vs unbypassed

 

[abbey road d enfer]

Why don't tube pres use the old fuzz pedal trick where the emitter / cathode resistor is actually a potentiometer with a large cap to ground on the wiper.

This type of NFB does not have some of the advantages of global NFB. In particular it doesn't improve the noise performance that global NFB brings by cancelling the noise of the subsequent stage(s). Also, this type of NFB increases the output impedance of the stage, which may be detrimental to the HF and transient response. It does not significantly reduce distortion either. It does not decrease the global output impedance, with all the consequences it may have on response.

 

[abbey road d enfer]

I suppose you could try that ,but my guess is that the amount of gain range you would get wouldnt be massive.
just found this ,
7db difference with 12ax7 bypassed vs unbypassed

It is actually possible to modify a common-cathode stage for a larger range of gain control, but that would be at the expense of increased noise and reduced headroom.

 

[Tubetec]

Thanks for the reply Abbey,
So maybe a sectionalised,PI wound ,or Bifilar ,
Maybe CJ knows

 

[ruffrecords]

Who here measures the performance of the equipment they build anyway.

I do.

Cheers

Ian

 

[ruffrecords]

In saying what I said above ,of course I dont mean to underplay the importance of proper scientific measurements and testing .However with simple tube and transformer coupled circuits analising about  THD numbers doesnt give the full story, the balance between lower and higher order harmonics is more important.

PC based audio spectrum analysers are cheap nowadays so there is no excuse for not seeing the harmonic structure. REW (free) married to a decent USB powered audio interface (like the Scarlett range for example) is all you need.

There really is no excuse for not knowing how a design measures.

Cheers

ian

 

[Matt C]

PC based audio spectrum analysers are cheap nowadays so there is no excuse for not seeing the harmonic structure. REW (free) married to a decent USB powered audio interface (like the Scarlett range for example) is all you need.

slight derailment, but Ian I'm wondering about REW - when I use it, the minimum output level seems to be -40dB.  What do you do to measure higher gain units?  Just put a simple pad in front of it?  or is there a way to adjust this in the software?

 

[ruffrecords]

slight derailment, but Ian I'm wondering about REW - when I use it, the minimum output level seems to be -40dB.  What do you do to measure higher gain units?  Just put a simple pad in front of it?  or is there a way to adjust this in the software?

This is one area where digital falls down. I have a little box I use. it takes the generator output and passes it trhough a600:600 transformer to protect the sound card from phantom power. Next comes a three way switch that selects 0dB, 40dB and 60dB pads for measuring mic pres.  This feeds anoother three way switch that selects the generator (voa the 0,40,60 switch), a short circuit and 150 ohms for measuring noise. On the input side is a 600 ohm load and a switchable 20fB pad so I can measure up to +20dBu output levels. I think I published the schematic of this box in  a thread in the Chamber.

https://drive.google.com/open?id=0B_n67A1hN3qtRmhlU3ZqUmMzdkk

Cheers

Ian

 

[moamps]

Thanks for the reply Abbey,
So maybe a sectionalised,PI wound ,or Bifilar ,
Maybe CJ knows

No doubt.
https://groupdiy.com/index.php?topic=36618.0

 

[Tubetec]

Yeah I got the REW alright , didnt really do much with it yet though .
Dug up a Varley LF Choke that I had lying around today , 75 HY @70ma ,two identical sections ,pi wound ,wondering if it would be worth trying the parafeed with it , apparently according to the Radio designers handbook its also possible to do parafeed with a cathode follower . I believe some of the super high end Audio Note stuff uses the parafeed with huge Hi-B cores costing thousands .
There are a few hitachi metglass cores and dual coil formers  available off ebay fairly cheaply ,comes unassembled ,looks like it would be easy to wind a superinductor ,not sure about the assembly, clamping and mounting of the cores though. Anyone with any experience with these? 

 

[Tubetec]

Cheers Mo,

 

[Matt C]

Interesting information, thanks.  Let me alter my question a bit: when you are getting a tube preamp like this to distort, are you better off creating the majority of that distortion in just one of the stages, or creating multiple points that are all distorting at once? Tonally, what are the implications of each?

A bit of background, a while ago I built a two-channel preamp based on the Collins 6Q-1, I love the distortion characteristics when it is cranked but I find it's a bit of a one trick pony. I'm aiming to build something that can give me similar (or even nicer?) distortion, but can also perform well as a relatively clean, high gain preamp.

The 6Q-1 is just two triode stages, 2nd stage is R-C coupled to the output transformer, B+ is quite low around 150Vdc. So maybe all I need is a similar topology with another tube and some clever gain staging options.  All these other arrangements just have me wondering what I'm missing.

 

[abbey road d enfer]

Interesting information, thanks.  Let me alter my question a bit: when you are getting a tube preamp like this to distort, are you better off creating the majority of that distortion in just one of the stages, or creating multiple points that are all distorting at once? Tonally, what are the implications of each?

It's almost impossible to answer this for you, because we don't know what you may or may not like. Distortion in the first stage is generally very different than that of the final stage, since the latter reacts with the output xfmr. Think guitar amps, where most players like the distortion of the output stage being cranked up whilst other are content with input distortion à la Mesa Boogie.

A bit of background, a while ago I built a two-channel preamp based on the Collins 6Q-1, I love the distortion characteristics when it is cranked but I find it's a bit of a one trick pony. I'm aiming to build something that can give me similar (or even nicer?) distortion, but can also perform well as a relatively clean, high gain preamp.

You may be on a wild goose chase here. Euphonic distortion in pro audio products is serendipitious; designers design for cleanest, and users abuse gear. I don't believe you can design a jack of all trades that you could dial for clean, crunchy or dirty at the flick of a switch. Just look at how there are so many different overdrive pedals and all have their use and idiosyncracies. In fact the closest thing to what you're asking is a DSP-based modelling preamp. I don't think that's the route you want to take.

The 6Q-1 is just two triode stages, 2nd stage is R-C coupled to the output transformer, B+ is quite low around 150Vdc. So maybe all I need is a similar topology with another tube and some clever gain staging options.  All these other arrangements just have me wondering what I'm missing.

You will have to experiment with all sorts of variations and topologies in order to find what you like or not, and live with the fact they're all different. That'll give you a good excuse for owning more than one mic pre. 

 

[Matt C]

Fair enough.  I know it's a pretty amorphous question, but maybe some other folks have found themselves in a similar situation and have found things that work for them.

 

[emrr]

What abbey just said.....

 

[ricothetroll]

High gain/distortion tube guitar amps like Mesa Boogie/Soldano generally get this kind of sound by multiple stages of 12AX7 distorting in the preamp section, and generally with attenuation between the stages in order that none is distorting too much. The other important thing, which in my opinion is what has the most dramatic effect on sound, is filtering before and after distortion stages. Note that we're not talking about slight crunch here...

Take a look at this project, might suit your needs :

https://groupdiy.com/index.php?topic=38252.0

Goes from quite clean to VERY dirty, can even be used a a guitar preamp/distortion.

One drawback though, for an extended bass response, you need a really big and expensive output transformer. The one Jonte Knif uses (Lundahl LL1671) costs 200€ per trafo. I used Carnhill VTB2290 which is 22€, and is still fine. See page 2 for measurements.

 

[bruno2000]

Who here measures the performance of the equipment they build anyway.

I do!  Eyes first, then ears..........
Best,
Bruno2000

 

[ruffrecords]

Check out the Rude Tube:

https://groupdiy.com/index.php?topic=49623.0

https://groupdiy.com/index.php?topic=45502.0

Cheers

Ian

 

[PRR]

> swing up to twice the supply volts giving 6dB extra headroom.

More than that. The SE choke/transformer power amp can be 25%-40% efficient in practice. An R-C coupled power amp is 5.8% at best, and with tubes usually under 3%. So for similar power and dissipation, the DC in the iron allows 10X power, 10dB.

But if the goal is "significant distortion", bah. Keep it simple, cheap, low power. Even a fairly "poor" design will output ample to clip your ADC inputs.

> choke is not in the signal path, which is utterly wrong.

+1.

The dead cats in my living room are not in the Signal Path from speaker to ear. But when I shovel them out, the room sounds very different.

> maybe PRR could chime a wise bell...

Maybe the coil-winder's guild needed more work/money so bribed the designer to double-up the iron.

There are a few use-cases.

If you select the C between the two L correctly you get a steep bass-cut with a possible peak at the corner. This seems to have been a way to compensate a row of transformer-coupled stages and their accumulated bass-slopes. If the early stages were down 3dB @ 50Hz, bump-up the output stage 3dB @ 50Hz and it all comes out near flat to below 50.

In extremely large systems such as radio transmitters, DC may be heavy and so is stray C, and design-time is available. Iterative computation of various winding techniques on both irons *may* lead to a wider-band solution than a single iron. The key is that leakage inductance is bad in the transformer but not-so-bad in the choke. (But the more you refine it, the more the choke LL finds C to resonate with and put resonances in the response.)

> I don't believe you can design a jack of all trades that you could dial for clean, crunchy or dirty at the flick of a switch.

That would be the Holy Grail of guitar players. Nobody has found it yet. There's at least 6 common popular topologies and 987 specific builds, mostly different.

I hate to say it, but I used to do my dirt in the DAW. I really can't accept the idea of distorting a live performance-- what if you over-do it? There is no un-do! Capture clean, then beat it in the CPU, with un-do.

 

[Matt C]

I hate to say it, but I used to do my dirt in the DAW. I really can't accept the idea of distorting a live performance-- what if you over-do it? There is no un-do! Capture clean, then beat it in the CPU, with un-do.

A few reasons:
1. I do a lot of analog recording, so I need some kind of hardware option.
2. Sometimes it feels good to just commit to a sound
3. I record a lot of very distorted music, and I hate being that engineer who is recording everything clean and having to say "I know this sounds nothing like you want it to sound, but we'll fix it later"

Whatever solution I find, I was planning on building it to be usable at line-level too, either just with input pads or whatever.
That will make it easier to fine-tune at mixdown. Sometimes I use the DAW for distorting stuff, but I can't rely on that as my sole option.