Want to build tube mic pre without buying kit. Suggestions.

[gibsonripper]

Don't transformers introduce phase shift as well, when implemented in the audio path?

I have right here in front of me a data sheet from a Jensen transformer I just used in a microphone build, and on the graph that shows "deviation from linear phase," it's up about 1.2 degrees at 20 Hz, with a gradual slope, linear from about 1k on up. And this is a nice transformer.

Capacitors have phase shift as well. Both do.

1.2% is negligible for most people.

I was just looking to eliminate one more part, not substitute a cap for a transformer. My issue was with eliminating the output capacitor feeding the output transformer (which the O/P XFMR is essential for impedance matching as well as voltage step ratio, too).

[Marik]

How much gain you want? If you need a lot, then you might need a few stages. If no caps is a must, then you could try something like Loftin White topology. The stability can be an issue.

Or you could use a choke loaded first stage direct coupled to CF, with more expensive transformer which allows DC on primary, then your gain stage tube would operate nearly its mu, won't lose your db's with high step down, and can use the pri of output as a part of cathode resistance.
In fact, this one can sound very, very good.

[gibsonripper]

Or you could use a choke loaded first stage direct coupled to CF, with more expensive transformer which allows DC on primary, then your gain stage tube would operate nearly its mu, won't lose your db's with high step down, and can use the pri of output as a part of cathode resistance.
In fact, this one can sound very, very good.

That's what I'm aiming for. Thanks for the reference. I'll look into it.

[nclayton]

It seems like the two biggest problems with using no caps is the need to use special gapped core transformers and making a volume control that's effective.

Lots of DC coupled tube circuits are heavy on cathode followers, but CF's don't sound as good as plate loaded because of the difficulty in applying NFB to them (despite whatever anyone says, CF circuits are 0% NFB).

what about something like this?



Since it uses a push pull output you could probably get away with a conventional transformer with maybe 10K primary center tapped. The cap at the bottom of the volume control is still technically a DC blocking cap, but it has the opposite effect of a normal series DC blocking cap in that if it's too small it will boost bass rather than cut it. best to make it large enough to be effectively ground at audio frequencies, but if you want you can decrease cap size to make up for LF lost in transformers if any.

Other drawbacks are the volume pot should be reverse log, and the output is obviously push pull which you may frown on, i can't remember if your goals at the beginning of this thread included no push pull.

Ned

[gibsonripper]

Other drawbacks are the volume pot should be reverse log, and the output is obviously push pull which you may frown on, i can't remember if your goals at the beginning of this thread included no push pull.

Ned

I'm going for a Class-A topology--therefore, I'm going to use caps in my pre-amp (just with a minimum of caps--meaning simplified circuitry).

I've got my design ideas and are implementing them this week. (may not be done for a couple months or so)

I'll post a new thread if/when I like the results.

Thanks for the feedback (no pun intended!).

[nclayton]

Cool. I'm sure you'll get good results with caps.

Just to clear up, though, the circuit I drew there would run class A, the output is just not single ended. In most cases class A doesn't have anything to do with topology, just with the particular values of resistors chosen. Then again, certain topologies (ie single ended ones) can only give usable results for analog when run class A, and I assume that's what you're talking about -- using a specifically single ended topology not just any old class A circuit. Just a nitpick, but it seems like there's some confusion (not necessarily on your part but in general) about the overlap in meaning between "class A" and "single ended".

Single ended and push pull are both topologies, class A just means there's more quiescent current running through the devices than will ever be consumed by the load, regardless of topology. You can tell the topology of a circuit just by looking at it's shape, but in general you can't always tell what class it runs in until you know actual values of components, supply voltages, and running conditions like load and level.

[gibsonripper]

Cool. I'm sure you'll get good results with caps.

Just to clear up, though, the circuit I drew there would run class A, the output is just not single ended. In most cases class A doesn't have anything to do with topology, just with the particular values of resistors chosen. Then again, certain topologies (ie single ended ones) can only give usable results for analog when run class A, and I assume that's what you're talking about -- using a specifically single ended topology not just any old class A circuit. Just a nitpick, but it seems like there's some confusion (not necessarily on your part but in general) about the overlap in meaning between "class A" and "single ended".

Single ended and push pull are both topologies, class A just means there's more quiescent current running through the devices than will ever be consumed by the load, regardless of topology. You can tell the topology of a circuit just by looking at it's shape, but in general you can't always tell what class it runs in until you know actual values of components, supply voltages, and running conditions like load and level.

When referring to audio circuits, Class-A always indicates single ended. Single ended, however, doesn't always assume Class-A. For all practical purposes, it is assumed to be single ended, because we're talking about tube circuits here. The circuit looks good--however, I would be fearful of making sure the tubes triodes are carefully matched. That could introduce non-linearities, and therefore unwanted distortion.

[???????]

When referring to audio circuits, Class-A always indicates single ended. Single ended, however, doesn't always assume Class-A.

Maybe i'm misunderstanding you, but isn't that the other way around?

It is possible to have a push-pull class A amplifier, but it is impossible to have a single-ended amplifier of any other class.

[gibsonripper]

When referring to audio circuits, Class-A always indicates single ended. Single ended, however, doesn't always assume Class-A.

Maybe i'm misunderstanding you, but isn't that the other way around?

Oops, yes. That's what I meant. Thanks for the correction.

I did some extensive researching and found that a DC servo would be the ONLY way to stabilize a xfmr coupled O/P. The counter DC would be something that could be a pain to implement. Some old Tektronix scopes (tube type) actually had a high-gain, DC stable circuit, but when it comes down to it--easier to stick with the simple, time-tested method.

Thanks very much for the correction and feedback--hope this thread hasn't been too annoying to anyone. I like to encourage new designs and ideas for high-end prp-audio equipment.

[nclayton]

It is possible to have a push-pull class A amplifier, but it is impossible to have a single-ended amplifier of any other class.

This is (I am) getting silly, but it actually is possible to run a single ended amplifier NOT class A if you're using it as a switch. Even with audio, technically if a single ended amplifier's allowed to clip on the high side it isn't running class A anymore since the tube isn't conducting for some part of the duty cycle. I realize all this is beside the main point, but I mention it to just hopefully back up my totally lame side point, which is that circuit topologies and current classes don't technically have any exclusive relationships with each other. Topology is the "shape" of the circuit and class has to do with the amount of current running in the circuit, so they kind of are related but not related I guess.

I just drew that quick sketch to show the basic "topology" I was thinking of without having to use words and stuff which I suck at. But if anyone was going to actually make an amplifier like that they'd probably want to put individual resistors on each cathode in the LTP to give some negative feedback. Letting one of the resistors be trimmable or using a trim pot as a "Y" between the cathodes would be OK ways to balance the DC current. I don't think a servo would be necessary. Obviously you want to get as close to 0 net DC as possible, but all EI core transformers have at least some distributed gap, and you usually don't have to be absolutely precise about trimming DC imbalances. The better the output tubes currents were matched the lower the LF distortion would be, but virtually every PP tube output ever made uses a "No DC" core, and obviously none have used any kind of servo, with the possible exception of recent hi tech ones.