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Cassiel 23rd March 2013 12:30 PM

Improving preamp's bass response.
 
2 Attachment(s)
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?

jan.didden 23rd March 2013 12:35 PM

Well, the low end is determined by 'Ck' and the output coupling 'Cap' value.
What are those values?

Edit I see: Cap = 2uF: with a 22k Ra: the Fc = 1/(2*pi*22k*2uF) ~ 5Hz.
Rk = 1k which is 22x as low as Rk so for the same 5Hz you'd want Ck to be like 50uF or better 100uF.


jan

DF96 23rd March 2013 12:39 PM

The output cap may need to be bigger - depends on the input impedance of the power amp.

Vinylsavor 23rd March 2013 12:39 PM

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

JonSnell Electronic 23rd March 2013 12:42 PM

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.

Cassiel 23rd March 2013 12:52 PM

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.

Vinylsavor 23rd March 2013 12:57 PM

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

Cassiel 23rd March 2013 01:37 PM

Quote:

Originally Posted by Vinylsavor (Post 3423824)
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.

DF96 23rd March 2013 01:44 PM

Quote:

Originally Posted by Harleyjon
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.

GoatGuy 23rd March 2013 02:34 PM

Quote:

Originally Posted by DF96 (Post 3423877)
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


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