Bass preamp considerations

[viceroyal]

I haven’t been on this site in quite a while. But to be honest I was never really an active poster. I got into DIY to build myself a bass preamp and that has taken me on a several year journey building tube amps and effects pedals. I’ve come full circle and am looking to design a new bass preamp for myself. I am a huge Leo-phile so most of my stuff has been Fender-ish in the past. I’m looking to go Ampeg-ish this time around.

I will be working on 2 broken V4’s in a week or so and if I can fix one I can keep the other! This has caused me to research the hell out of these amps. I’m really interested in the preamp design and I plan to make a scaled back version for myself. I do not have a schematic drawn yet but if you are familiar with it I plan to copy from the input to right before the phase inverter. I haven’t decided how I’m going to approach the active mid control yet but that is not really my concern at the moment. I have 2 other design problems I’ve been asking myself and I thought I would see what you guys have to say.

Design Query Number 1:

If you are familiar with these amps then you know that there is a buffer after the active mid that goes to a slave amp jack and then after that there is a final gain stage before the power amp. My initial thought was to remove the buffer since I won’t be needing an output before the final gain stage but then I thought about looking into reasons I may want to keep the buffer in that position. I don’t need another gain stage and I don’t want to waste a triode so leaving it as a unity gain buffer will keep me from having to deal with far too much gain. One thing I saw mentioned but I can’t seem to find it again is that having a buffer between gains stages can help to avoid blocking distortion (which can be a problem when dealing with bass guitar). Is there any truth to this? Any other benefits to having a buffer between gain stages (besides when needing one before a tone stack)?

Design Query Number 2:

Also, I’m looking to experiment with saturating an interstage transformer (or mic or output transformer used as an interstage transformer). What sort of specs should I look for to find a suitable transformer? Would I even be able to here it saturating? Will it sound good? I know some mic preamps are pushed hard to overdrive input transformers. I guess this would be easy to do with the voltages coming out of the tubes. Is there anything else to consider. Would this even work?

Thanks for reading!

[loudthud]

The unity gain buffer is needed to isolate the mid control from loading. If you are driving a high impedance and no cable, leaving it out shouldn't hurt anything.

I can't comment on the saturating transformer idea but there is a guy from Russia that designed a tube power amp emulator on the Solid State Guitar Forum that you might want to look at. He wound his own transformer.

I had a thought about the inductor that would be needed for this type preamp. You can probably find a cup core and wind one with guitar pickup wire. This could also solve the problem of the guys building Mesa MK-II amps that need inductors for the equalizer. Just need to find the source of a core that would work.

[exclamationmark]

You can still get blocking distortion quite easily even when using buffered inputs if they are capacitively coupled to the tubes. Direct coupling solves this issue quite easily. I've built a bass pre-amp out of a single 12ax7 tube, with the gain stages direct coupled to each other, and it worked out quite well. Really, even for bass I don't think direct coupling is really necessary, as you still try to minimize the size of the coupling caps (or the bass response). Distorted bass sounds kind of terrible in my opinion (like in the 50-300Hz range), and I will generally try to have a clear low end, with the dirt sort of riding on top. Then again I'm primarily a guitar player. YMMV

Which brings me to my second point. I've actually built tube amplifiers without output transformers, and the funny thing is... the same circuit with a transformer sounds pretty much exactly the same. I think transformer saturation is just some sort of tube purist perpetuated myth to sell amplifiers. I've never seen anything that looked like heavy saturation when testing the output power into a resistive load. There's a bit of crossover distortion, but I'm fairly sure that's just from class AB operation.

Saturation looks a lot like cross-over distortion, and I think would probably sound even worse, as saturation really only occurs on the lowest frequencies. I've even gone so far as to model saturation using a fake hysteresis curve on matlab. It didn't sound very good. Though that may have been down to my horrendous math skills, the output looked remarkably close to what you would expect in real life. Another thing to be aware of is that a transformer undergoing saturation will get very hot.

With that being said, I've never heard transformer saturation 'in real life', and wouldn't have any idea how to get it from an interstage transformer (other than undersizing the thing to a ridiculous degree).

[Steve Conner]

Ah. transformer saturation.

I did a series of experiments on this, using a couple of tube amps I had handy, and couldn't see any saturation at guitar frequencies.

I think what happens is, when a push-pull tube amp is overdriven, the two sides of the output stage can clip with different duty cycles. Or the duty cycles are the same, but the peak currents are different because of unmatched tubes. Either way, the average currents in the two sides could be different, the result will be a DC imbalance in the output transformer, and that's what saturates it. Not the volt-seconds of the signal waveform.

I have a single-ended amp with power scaling that varies the screen voltage, and so the plate current of the power tube. You can really hear the effect of the small, cheap output transformer going into saturation at higher plate currents. The bass and treble are cut back and the sound is focused into the midrange. That happens for any volume level, because it's an effect of the DC bias.

I suspect that DC imbalance in an overdriven push-pull amp does the same thing, except it sounds more exciting because it's modulated by the signal.

[Gtr_tech]

A V4 (in proper operating condition) makes a hell of a bass amp all on its own. Why build what you already have?

[viceroyal]

A V4 (in proper operating condition) makes a hell of a bass amp all on its own. Why build what you already have?

This is an excellent point but I doubt I need to explain to you guys that sometimes the journey is the more rewarding part! I also don't think I need to explain that one amp is never enough! I find myself in several different playing situations as of late. The V4 will stay at the studio but the new preamp would be able to travel with me for at home jams and one off jams with friends.

I appreciate all of your responses. I'm thinking I'll just use three gain stages then the last triode as an output buffer. I'm all about keep the low end in check (I have a tube-based bass fuzz the rolls of at 70 Hz on the input). I was just hoping to let some sub lows through this time for when I downtune for my metal band. That's why I was concerned with blocking distortion.

[Gtr_tech]

Well good luck finding that torroidal coil in the mid circuit...

[J M Fahey]

I have built "Ampeg" tube preamps and , of course, wound my own toroids.
Had to do everything, even the rack cabinet, the front panel, silkscreening, the full Monty:

Here you can clearly see the 3 tap handwound toroid:

Here you see it sitting in the rack, together with another of my tube preamps, this one an "Orange" type.
It´s driving two Rack PA power amps, an AB1500 (W) and a QSC950(W), driving 4 Ampeg 8x10" "fridges".
My customers do play LOUD.
Well, they regularly play 50000 seat Football (you call it Soccer) Stadiums in Argentina and surrounding Countries.

The unplugged Ampeg SVT 400 you also see in the rack is the backup amp, in case of any problems.
A front panel closeup:

[viceroyal]

I have built "Ampeg" tube preamps and , of course, wound my own toroids.
Had to do everything, even the rack cabinet, the front panel, silkscreening, the full Monty:
[ATTACH=CONFIG]17690[/ATTACH]
Here you can clearly see the 3 tap handwound toroid:
[ATTACH=CONFIG]17691[/ATTACH]
Here you see it sitting in the rack, together with another of my tube preamps, this one an "Orange" type.
It´s driving two Rack PA power amps, an AB1500 (W) and a QSC950(W), driving 4 Ampeg 8x10" "fridges".
My customers do play LOUD.
Well, they regularly play 50000 seat Football (you call it Soccer) Stadiums in Argentina and surrounding Countries.
[ATTACH=CONFIG]17692[/ATTACH]
The unplugged Ampeg SVT 400 you also see in the rack is the backup amp, in case of any problems.
A front panel closeup:
[ATTACH=CONFIG]17693[/ATTACH]

These picks are awesome. Did you or your customer feel like these preamps captured any part of the sound of a full tube Ampeg? Or was it just a tube pre with a sound all of its own? I know I'll be missing the heft of the power tubes but I'm hoping to get a sound similar to turning a V4 or SVT down to practice levels.

About the inductor, I have some larger ones I was going to use for a Bench style EQ so I'm going to pick one of the middle frequencies and try to make one of them work. If not I could most likely find at least one online somewhere that will work. If that becomes too troublesome then I could try a simple gyrator circuit. Like I said in my first post, I haven't figured out how I'm going to do the midrange yet. I could also either leave it out or since I have so much gain on tap I could have a preset passive mid boost. There's enough range in the James stack to even it out if need be.

I'm buying some aluminum stock this week and I plan to build a new prototyping rig so I can tweak it to taste before committing it to its own chassis. Plus, I can use that preamp test board for future projects.

[J M Fahey]

Thanks.
I'd say it's 80% of a *Clean* Ampeg, which is very good.
Same as hooking what they call "External amp out" into a recording or PA board.
What it really lacks is tube compression.
A Rack studio compressor/limiter *can* be adjusted to get the extra 20%.
My customer really had way too much clean power (1500+900W RMS) so he never even approached clipping.
You might add a small 1 x EL84 stage at the end into a 10 ohm 10W resistor and reamplify that crunchy signal.
Forget the gyrator, you need a real inductor.
Part of the Ampeg sound is the "Midrange growl" appearing when a loud signal hits the maxed midrange control.
An SS gyrator and its +/-15V rails can never be compared to a tube fed 250/300V.

[Tubebass]

Thanks.
I'd say it's 80% of a Clean* Ampeg, which is very good.
Same as hooking what they call "Exyernal ampP into a recording or PA board.
What itreally lacks is tube compression.
A Rack studio compressos/limiter can be adjusted to get tha extra 20%.
My custoner really had way too much clean power (1500+900W RMS) so he never even approached clipping.
You might add a small 1 x EL84 stage at the end into a 10 ohm 10W resistor and reamplify that crunchy signal.
Forget the gyrator, you need a real inductor.

Didn't SWR do the EL84 thing in their "Interstellar Overdrive" preamp?

[viceroyal]

Yup and H&K did it with the BATT preamp. It's something I've considered but haven't experimented with.

[viceroyal]

Thanks.
I'd say it's 80% of a *Clean* Ampeg, which is very good.
Same as hooking what they call "External amp out" into a recording or PA board.
What it really lacks is tube compression.
A Rack studio compressor/limiter *can* be adjusted to get the extra 20%.
My customer really had way too much clean power (1500+900W RMS) so he never even approached clipping.
You might add a small 1 x EL84 stage at the end into a 10 ohm 10W resistor and reamplify that crunchy signal.
Forget the gyrator, you need a real inductor.
Part of the Ampeg sound is the "Midrange growl" appearing when a loud signal hits the maxed midrange control.
An SS gyrator and its +/-15V rails can never be compared to a tube fed 250/300V.

I'm glad you added this last line. I asked an engineer buddy of mine if there were any reason to prefer inductors over gyrators and he said they could be made to sound the same but couldn't offer a reason to choose one over the other. I was coming here to ask why you suggested a real inductor. I know studio guys prefer them for "vintage" and "warm" sounding EQ's but honestly those two words aren't enough to convince me.

[exclamationmark]

I've actually replaced my EQ in a guitar preamp with pseudo parametric one and it works wonders! (though to be fair, it does give a lot of useless combinations). It just consists of your standard high pass/low pass filters for the treble/bass controls, but with a parametric EQ for the mids. You can adjust the Q, the centrepoint frequency and the amount of cut/boost. When I used a bax stack, I always used to set it so the frequency response was dead flat (without even knowing it) because everything else sounded rubbish. The parametric mid control was born out of frustration with the bax stack, and the inability to decently control mids. Just by notching out a certain frequency band (or boosting), it changes the whole character of the amp!

The ability to dynamically change the parameters of the simulated inductor is probably one of the gyrators strongest points, making them insanely useful. Parametric EQ's would be extremely difficult to construct without them (though adjustable inductors do exist). I've used gyrators in SMPS power supplies to remove the HF hash as well, in place of an inductor the size of my fist. If you use poly caps, a gyrator will probably act more like an idealized inductor compared to an actual inductor! Conversely, their resonant properties are limited by the maximum voltage applied to the active circuitry (ie, no LC tank circuits). I don't actually know if they respond to transients like inductors do, either.

[Steve Conner]

And remember you can make high voltage gyrators too! Use a MOSFET or a high voltage BJT like the MJE340.

The inductor will generate distortion because of its non-linear magnetic characteristics. Maybe this is what JM is thinking of.

Unlike a real inductor, a gyrator can't store energy. It uses energy from the supply rails to give the illusion of doing so. Using one as a SMPS filter inductor makes no sense. It would be a linear post-regulator at best.