Preamp EQ

[HariGanti]

I am thinking about building a hybrid amp, with a valve/tube preamp. One of the features I wanted was tone control, and I'm trying to decide between three approaches:

First is a tone stack, where the output of the tone stack goes into the preamp tube.

Second is splitting the source signal into separate signals, then using a bandpass filter on each of those to select for certain frequencies, then finally sending the output from the filter to the preamp tube.

Third is something I've thought about, but I'm not sure will work. Instead of changing the source signal at all, I was wondering about using a bandpass filter on the decoupling part of the cathode-bias circuit on the preamp. Instead of just decoupling with a capacitor, I was thinking about a shunt (or parallel) LC circuit that acts as a specific bandpass, which should change the frequency response of the tube.

Does anyone have any experience with any of these, or any good ideas about them? Thanks!

 

[Loren42]

If this is for HiFi and I were to do this I would use tubes for the power amp and ICs for the preamp.

The reasons for this would be:

1. Tube power amps manage changing impedance of speakers in a way that provide more power as impedance goes up. This will give you a little more bass and brighter treble depending on your speakers and their impedance curve (slightly warmer, richer sound).

2. Tone networks add the potential for more hum and noise since you need more gain to compensate for dB losses in the tone network. Using tubes is the hard way with no real sonic advantage, whereas a well designed solid state IC has lower noise and distortion figures. So, using multiple stages and tone networks with ICs presents less problems.

Additionally, with ICs you can use fewer capacitors (coupling caps) in the signal path, which reduces phase shifting.

Your reasons for doing what you propose are up to you in the end.

 

[skidave]

Here is a link to a tube tone control I built. I use it with my tube preamp, but it is in a separate enclosures. The information in the link should give you all that you need.

Dave

diytube.com • View topic - Tube Tone Control Project

 

[HariGanti]

Loren42, I've actually decided to use one of the tone control schematics from skidave's link, since the two other methods I was thinking of are simply far more complicated than I'd want to get into. Still, with tube vs SS for the power section, I am wondering how, specifically, the tube compensates for a changing speaker impedance. If, for example, you have a pair of 4 Ohm speakers, then as the frequency changes, so does the impedance (even if only slightly). How does a tube compensate, and is there circuitry that allows transistors to dynamically impedance match the speaker? Is this still a problem if the output goes through a transformer first, since now the source impedance is, I believe, the impedance of the secondary winding? Thanks!

 

[DF96]

On average valve amps have a higher output impedance than solid-state amps. Some valve amps have a particularly high output impedance, which leads to a boost at the speaker bass resonance. Not hi-fi, but some people like this sound.

For most speakers and most listeners the aim should be to avoid this effect. This is normally achieved by using negative feedback; some combination of local and global.

 

[HariGanti]

DF96, would this feedback be in the preamp section, or in the power amp section?

 

[HariGanti]

Also, what is the typical amplitude used in a preamp stage? My computer appears to be able to drive (at max) about 150mV into a 10k load at 10Hz (I needed a really simple test...), but I doubt that's enough of a swing. I was thinking of a small pre-preamp stage to multiply the voltage by a factor of ten (1dB gain). I was thinking of using a MOSFET or BJT for that.

Also, any thoughts on BJTs vs MOSFETS for a power section? I know this forum is all about tubes, but I figured I might as well ask.

 

[DF96]

To get a low output impedance the feedback has to be taken from the output, as most power amps do.

A voltage gain of 10 is 20dB.

I'm not sure what you mean by 'tone stack'. If you mean the passive tone control often used in guitar amps, then this is best avoided in audio - it just adds noise. Better to use an active tone control e.g. Baxandall.

 

[HariGanti]

My bad with the dB, I actually mean that number in bels. Isn't 10 dB or 1B a factor of ten, since it is logarithmic?

Anyway, I am using an active tone control scheme. It follows a cathode follower tube and precedes a common cathode tube.

One other question I had was about the anode voltage. What would happen if I were to raise/lower it during operation? Shouldn't that change the effectively change the gain and the distortion of the tube? Has anyone tried this, or is there a *really* good reason to avoid this?

 

[DF96]

10dB is a factor of 10 in power. You said a factor of 10 in voltage, which is 20dB.

Anyway, I am using an active tone control scheme. It follows a cathode follower tube and precedes a common cathode tube.

You do understand that these two sentences don't say the same thing? They don't clash, so they could both be true but the second one does not imply the first one. I only raised the issue because 'tone stack' appears to be a term used only in guitar amps. Hi-fi may have 'tone controls'.

One other question I had was about the anode voltage. What would happen if I were to raise/lower it during operation? Shouldn't that change the effectively change the gain and the distortion of the tube? Has anyone tried this, or is there a *really* good reason to avoid this?

The really good reason to avoid this is that it changes the gain and distortion. OK if you want an effects box. Not what you want for hi-fi.

 

[HariGanti]

10dB is a factor of 10 in power. You said a factor of 10 in voltage, which is 20dB.

My bad. I completely spaced on that.

You do understand that these two sentences don't say the same thing? They don't clash, so they could both be true but the second one does not imply the first one. I only raised the issue because 'tone stack' appears to be a term used only in guitar amps. Hi-fi may have 'tone controls'.

Yes, I understand that they don't say the same thing. The second part was meant to elaborate on the control scheme, in case you thought the extra information was useful. Also, I apologize for my terminology. A lot of what I've read on amps has been about guitar amps, so I've picked up that terminology. I suppose that it is, correctly, a tone control system.

The really good reason to avoid this is that it changes the gain and distortion. OK if you want an effects box. Not what you want for hi-fi.

I agree that it will change both gain and distortion. In fact, that was part of the reason I was thinking about this. If I wanted to accurately reproduce the exact waveform that the signal had, but at higher current, wouldn't I simply want to use a full SS amp and keep the gain low enough to prevent clipping? Using tubes at all will inherently distort the sound. I was just curious since the ultimate goal is to achieve the best sound quality I can get, so if that means changing the distortion/gain, then I'm fine with that. This amp is something of an experiment anyway. Perhaps in the future I'll make a fully SS amp that is designed for hifi use, but for now, this will do. I'm glad to hear that it isn't fully unreasonable.

Thanks again for helping me with this.