-
Please note that there are a few updates and clarifications made in the Audiokarma Rules, mostly relating to advertising and the addition of the new "Paying it Forward" & "Giving back" forums in the AudioKarma Audio Marketplace section.
Recs for a tube preamp circuit
- Thread starter Remy K
- Start date
With mention of the 27, 37, and 76, there is a very good sounding preamp that has been around for a long time designed by Bruce Berman. It uses the very inexpensive 76 that Eric Barbour and Morgan Jones have found to be just as linear as the best 6SN7s.
Last edited:
Lavane
Lunatic Member
I agree. You can get NOS 6J5's cheaper than new manufacture or NOS 6SN7's. Personally I think the 6J5's sound better too. They're very linear tubes. I used some RCA metal 6J5'S and glass bottle 6J5's and preferred the metal ones. I found them to be very clean clear sounding tubes.
atmasphere
Active Member
The disadvantages are several. The circuit can be overloaded by a source. With the control at the input that won't happen. The control at the input will not affect the distortion signature of the circuit as well. Finally, a control at the output results in a variable output impedance. At less than full volume, bass can suffer. You can also get more colorations from interconnect cables driven in this manner.
Grid stopping resistors are advised. 1K will do nicely. You may find the result is the circuit to be more relaxed and lower noise despite the series resistors. We use a circuit much like this in our UV-1 preamp. Its a variant of the circuit we've been installing in PAS-3s since the late 1970s which used a 12AU7 since the PAS3 had so little current available from its power transformer.
A nice advantage is you can't come anywhere near clipping a circuit like this while driving an amp. So distortion is quite low and bandwidth is nice and wide.
If the preamp in mind is intended to be stereo, a balance control of some kind is likely a good idea. A shunt style balance control is simple to construct. The best place to insert it is before the volume control, since its wiper is grounded and is common to both channels. We use a 1M linear control with 49.9K in series with the input so one resistor feeding each side of the pot. That is then the same connection that feeds the top of the volume control. This reduces gain a bit but I think you'll find you have plenty of gain so no worries.
If you are hand-wiring, to avoid hum pickup the input ground, volume control ground and cathode resistor ground should all be connected together with a single wire that goes to the ground of the power supply. If you use the shunt control idea, ground the inputs separately. For a circuit like this which has a robust sound, the ODAM caps from VH Audio work quite well. We use a 2uf part at the output in our circuit but our power supply filter cap value is 100uf. If the coupling cap value winds up with a timing constant lower than that of the power supply, Bad Things can happen.
I've built at least a dozen of these for people and they love them. I used one to drive my KT120 mono amps and they had great bass response. I explain to folks to set the output at 1/2 to 5/8 and then adjust the trimmer pots to get normal "loudish" volume, the trimmers end up being at 1/4 or less. With the volume knob on the input, at low volume or possibly even at normal levels if the amp.had a reasonably sensitive input, you lose all the tube sound from a tube preamp.
I like the idea of that 76 amp, it's sort of exactly what I was looking for.
I was wondering how much I could get away with reducing the cost of power supply. At the moment two 15H and two 12H inductors would be a third of the price of the whole thing... Wondering how much they are necessary, of course ripple matters a lot here but maybe not that much. Maybe I could get away with two total, and some larger caps? or more cascaded rc circuits?
I was wondering how much I could get away with reducing the cost of power supply. At the moment two 15H and two 12H inductors would be a third of the price of the whole thing... Wondering how much they are necessary, of course ripple matters a lot here but maybe not that much. Maybe I could get away with two total, and some larger caps? or more cascaded rc circuits?
The above design focuses on isolating the two channels from each other, and that increases cost considerably. I wouldn't personally find that necessary, nor do I think 400V B+ is very useful.
The old Akai reel decks have monoblock amplifiers, each using a CLC power supply consisting of 20uF - 8H - 20uF. These supplies each power a complete SE 6BQ5 amplifier plus a head amp. They're very quiet. There are some inexpensive 7H chokes on the big auction site that would do the job, and good quality 33uF caps are inexpensive.
There are a lot of ways to skin this cat. You don't need to spend a lot.
Jack
I almost didn't include the PS schematic because any competent 400V supply will do. Personally I like to use inductors of low resistance because the voltage drop across an inductor of high resistance will cause the B+ voltage to sag when there is an increase in the current drawn from the amplifier circuit. With this preamp, that will be a very small amount because the tubes are biased class A and they are drawing minimal current to begin with. In a stereo preamp using the 76 and 6SN7, it should be <25mA.
I built a similar circuit for my PAS back in the late 1970s as well, It was suggested by a letter to the editor in Audio Amateur magazine. I cut traces and jumpered the Dynaco circuit board.
Last edited:
atmasphere
Active Member
I read the article. Not buying it. It is true that if the control is at the output the noise is reduced with the setting of the control. That's really the only advantage. But otherwise its all down side as I previously explained.
Regarding the claim of the distortion character, if you put the circuit on an analyzer you'll find the distortion increases linearly (or very nearly so) with the input Voltage. What this means is if the volume control is at the output rather than the input the distortion will vary from source to source depending on the output level of that source, some of which might drive the circuit quite hard.
That means some sources might seem to just not sound as good merely because of their output level, not just because the circuit is making more distortion, but also because the resistance in series with output coupling capacitor is increased.
If the volume control is at the input these problems are eliminated. Its easy enough to find tubes that are quiet enough the circuit can be well behaved even on horn speakers.
If variable distortion is desired, varying the cathode resistor value of the input gain stage would move the operating point across the load line thus achieving the goal. That might be a good way to set up a guitar effects pedal; if so I would use a variable light source and LDR so the control operation would not be noisy.
Is volume control at preamp output not the same thing as a volume control on power amp input? Seems like a lot of power amps have that.
I don't think I care or have interest in "coloring the sound," I just want a preamp that sounds clear and linear, with low distortion. I really like solo piano, and I find that too much "color" really messes things up. Just my opinion. And I don't have a volume control on the power amp.
I will look into putting together a power supply.
I will look into putting together a power supply.
This is the purpose of the input trimmers that @StepheK mentioned.
There are numerous ways to introduce distortion.
Most of us would probably agree with that. The product being discussed clearly serves a niche market.
Jack
Then why are you even looking at tubes? There is zero advantage to using tubes if what you are looking for is what you stated here. Possibly just a passive control is the best option unless you actually need "pre-amplification" gain.
Most people pining for a pre-amp today don't need one, and just want to add another component to the system for show.
Because building tube amps and listening to them is fun. I have had tube amps and solid state amps. Both can be very good, but I will not make arguments for the objective superiority of vacuum tubes. I don't believe in it. I'm sure there is something to the 2nd harmonic argument, but if that really was it we wouldn't be here, because there are numerous other ways to recreate that. I like tubes because I like old things, not because I am willing to die on the hill of them having some sort of technical purity.
Well that's a post I 100% disagree with.
Tubes are inherently more linear than transistors. And "linearizing" transistors is more difficult than tubes, because techniques like degeneration and feedback tend to create additional artifacts that often don't appear in typical distortion measurements. The end result is that it's extraordinarily difficult to create a solid state preamp or power amplifier with the transparency of tubes. Even a run-of-the-mill tube signal chain will best 99% of the transistorized gear that's ever been made.
I couldn't disagree with this more. I build hi-fi gear with tubes because they're more transparent and lifelike, not because they add anything to the sound. What you're building is in fact an effects unit, not a high fidelity product. As for preamps, many have features that cant be implemented with a passive design, such as tone controls and the ability to drive longer cables and lower-Z amplifiers without signal degradation. Other advantages are also possible with an active preamp, such as the consolidation of all control and gain functions (including phono preamplification) into one unit.
Jack