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Improving preamp's bass response.

Cassiel

Disabled Account
2004-09-30 3:53 pm
Madrid
Hi folks, my preamp does a little bit of magic with the mids (great for Jazz, acoustic music, female singers) but the bass is kinda soft and not punchy. Do you know any tricks to improve bass response? Compared to a Bryston and a Mark Levinson's it lacks a little in that department. Even though it beats both of them in musicality, noise figure and listening fatigue.

Power supply is the standard CRC filter with a tube rectifier (80). I listen to some ambient, electronic music and, even though it sounds great, the bass is obviously softer, the SS preamps have better bass. My plan is to change the topology of one channel to have the other as a comparison point. But maybe I only have to change the value of the output cap......or maybe add a regulated power supply. Just asking for some advice. Is it worth to try another topology or I should focus on the power supply?
 

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Hi!

Did you measure the frequency response? Before doing any modifications it would be a good start to first know where the low frequency roll off actually is. Do the measurement with an appropriate load resistor on the output which should be the same as the input resistance of the power amp your are using this with.

If the low freq roll off is too high, start with the cap values as Jan suggested above

Best regards

Thomas
 

Cassiel

Disabled Account
2004-09-30 3:53 pm
Madrid
Thanks guys. Rk is 470 uF. One 6SN7 per channel (parallel triodes). I will start with the changes described here - bigger output cap and, also, that cap in the feedback loop and see what happens. I don't have the equipment to measure the frequency response, alas, I'm just a tinkerer - far, far away from being a pro.
 

Cassiel

Disabled Account
2004-09-30 3:53 pm
Madrid
Hi Cassiel,

This will give you a lot of insight and help to understand things.

Thomas

No doubt about that Thomas. I used to rely on my sense of hearing coupled with a balanced intellect but it sometimes plays me tricks and, besides, it's not as good as it was. Still, that electronic bass should sound louder. Don't need a machine to tell me so. Soldering iron is hot now, let's see (hear) where it gets me.
 
Harleyjon said:
Make Ck 220uF 16Volts and connect the feedback to the anode through a separate capacitor, around 2n2 400 Volts, leaving the coupling capacitor at a value of 220nF 400Volts. With R1 loading the output, the bass roll off will be horrible but if fed by a 2n2 cap as described, should improve the low end gain.
This is essentially a bass boost tone control, but fixed. In my opinion, swapping components or changing the circuit is not a convenient route to tone control ever since potentiometers were invented.
 
This is essentially a bass boost tone control, but fixed. In my opinion, swapping components or changing the circuit is not a convenient route to tone control ever since potentiometers were invented.

LOL... I've long been favoring the school of "potentiometers everywhere" ... they're so darn helpful for experimentation!

Seems to me that the ONLY way the original amplifier has a lack of bass response is if the actual load downstream is substantially lower in A/C impedance than the listed 100K. In which case, just a somewhat higher value output cap is required.

Clearly the Ck/Rk combo is right-sized.

GoatGuy
 
Hi Cassiel,

no pro equipment needed really. I would highly recommend to get a scope and a signal gen. Used scopes can be found for small money. Signal generators are cheap too. For something like 200-300 Euros you can get both a scope and generator. This will give you a lot of insight and help to understand things.

Best regards

Thomas

I agree.

Even downloading the free HolmImpulse software for your computer and a cheap soundcard with a mic input will do the trick.

HolmImpulse will do a complete sweep across the audio spectrum in about a second and display the results as a graph on your PC.

You need to establish a baseline before making changes so you know where you are at and what changes do what. Subjective listening tests are not the way to go.
 

Cassiel

Disabled Account
2004-09-30 3:53 pm
Madrid
HolmImpulse

I have installed the program and I reckon it will take me a year to understand how it works.:)

You need to establish a baseline before making changes so you know where you are at and what changes do what. Subjective listening tests are not the way to go.

My baseline is a great sounding preamp with zero noise but (there's always a but) bass response could be better. And about what changes do what... I can say that doubling the value of my output caps did nothing. Anyway, I'm fed up - I hate this A/B thing. Sometimes the changes are obvious and easy to spot but when they're not... it's frustrating. It's like looking for a missing person. I think I'm gonna hire a detective (oscilloscope) to look for my missing bass. Either that or accept you can't have it all.
 
say that doubling the value of my output caps did nothing

I think you're "hearing phantoms" and preferences. The cathode cap (47uF) gives a -3dB rolloff of 6.3 Hz. Super. Not the problem. The output cap (2 uF) driving a 100K load, 0.7Hz. Clearly low enough!

Before you go all oscilloscope crazy, let me ask, what again is the point in even having a preamplifier that has a thru-gain of 1.28x ??? The best answer would be, "to convert from high-Z source impedance to low-Z input impedance of my amplifier" Because otherwise, there isn't much that the preamp with a gain of 1.29 is actually doing.

WAIT! I think I may just have gotten "it"!

This preamp circuit is a phase INVERTER. When your speakers are supposed to be punching OUT, they're now going to be sucked IN. And vice-versa. This very well could be the problem, in a nutshell.

Simplest test in the universe: reverse your speaker cable polarity. Well, not simple if you have some kind of standard connector other than banana-plug. But this would do the trick to test the polarity issue. If your amplifier is worth anything at all, it will symmetrically amplify the signal, so it itself doesn't "care" what the signal looks like coming in.

IF this fixes things, then there are two fairly obvious "fixes" that come to mind.

1. Feed anode of U1 to grid of U2, which has a pair of 22K resistors, one on anode, one on cathode in the classic "phase splitter" configuration. Tapping the plate with the output DC blocking capacitor as in your original drawing will deliver a PHASE NORMAL signal to your amplifier. Feeding the cathode (through a DC blocking cap) through the R2 NFB resistor preserves the basic circuit topology and NFB idealization.

2. ... or get a small interstage coupling transformer, and replace both the 22K plate load resistor and the output capacitor entirely. The beauty is ... if you get a center-tapped type, you can have both the positive output and the negative feedback at the same time. No additional tubes. Capacitor phobic types will see this as a great gain. You do have to have a nice transformer though, to get down to the 20Hz listening ideal.

There you go. I don't think putting bass "shaping" circuitry (DF96's tone control comment) is anywhere near as important as just re-reversing the phase to be coherent with the original sound sources.

GoatGuy