lack of bass in a 6h30 design - diyAudio
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Old 17th September 2006, 07:46 PM   #1
SunRa is offline SunRa  Romania
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Default lack of bass in a 6h30 design

Hello!

I've just finished a 6h30 srpp line stage. The schematic is here, and it's probably well known: 6H30s Line Preamp

The preamplifier is intended to be just the first from a series of designs, so it's meant for experimentation, in the search for the sweet spot for this tube. I must say it sounds quite great, very detailed, very intersting, a great soundstage but...
... it lacks bass. I have for now a poor signal source (my pc ), but still the bass is far better when the signal is taken directly. I've changed the 0.47uF philips output cap to some 1.5uF MKP and the bass significantly improved, but still not enough compared to the pc...

The power suply i use is a 2*300V, with a full wave configuration (2 fast recovery diodes, 75ns) followed by 2k2+33uF+1k8+470uF+1k8+470uF. The main reason for using all these resistors was due to the need of a high drop in tension (down to 150V, instead of 160 in the schematic).

Some other modifications are the lack of the 470uF filter cap presented in the schematic, and 180r instead 170r in the schematic.

Altough I plan next a mu-follower using a triode as ccs (modified srpp), or even a design with 10m45 like this one: http://blog.diyzone.net/blog/coffin/.../schematic.jpg but for now I am intrigued by this lack of bass... Oh... and yes... the 6h30's are brand new, with no burn-in... So.. some help would be great here...

Thank you in advance...
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Old 17th September 2006, 07:57 PM   #2
kevinkr is offline kevinkr  United States
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You didn't mention what kind of power amplifier you are using or its input impedance. Chances are if it is solid state it could be as low as 10K ohms.

The caps on the output of your sound card are likely to be 10uF or more.

Changing your pre-amp's coupling caps moved the low frequency corner down, but perhaps not enough.

I'd try something on the order of 3.3uF - 5uF. You could just add another 1.5uF in parallel with what you have got. Possibly shunt these with a good smaller film cap in order to maintain good high frequency performance.

Keep going until the bass is what you expect or you run out of space for the caps.
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Old 17th September 2006, 10:49 PM   #3
cerrem is offline cerrem  United States
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I agree with kevinkr...
His recomendation of larger output coupling caps seems to be in order.... 2.2uF to 5uF seems like a good range...
To determine the appropriate output coupling cap...you need to know 3 basic numbers...
1. The output resistance of your circuit...(SOURCE RESISTANCE)...

2. The input resistance your driving...(LOAD RESISTANCE)...

3. The -3dB cut-off frequency...( Low Frequency POLE).....
I typically use anywere from 6Hz on down....Since you want descent amplitude at 20Hz...also very important is to minimize the phase shift at 20Hz.....

For example:
Lets say we want fr=6Hz for the low frequency POLE, 500 ohms for the SOURCE RESISTANCE and 10K for the LOAD RESISTANCE....

First thing to note is the insertion loss, at mid-band....
This is = 20LOG[load/(load+source) ] .....
20LOG[ 10K/(10K+500)] = -.4237dB , this is insertion loss due to source resistance and is you mid-band 0dB reference to compare your POLE to.....as this source resistance approaches zero, so does this insertion loss... if your source and load were the same..then you would have a -6dB insertion loss..

To figure your Cap size....
C= 1/ [ 2*pi*fr*(Rload+Rsoure)]
C= 1/[6.28*6Hz*(10K+500)] = 2.52uF

Now just for example lets 'assume' your numbers are the same and you put in a .47uF Cap as in your case....
-3dB frequency= 1/[6.28 *.47uF *10500]= 32.26 Hz !!! Not too good!!!

The next step is calculate the output Resistance of your SRPP circuit and calculate out your values.... YOu can verify all this with scope and sig gen...

Best Regards
Chris
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Old 31st January 2007, 09:54 PM   #4
SunRa is offline SunRa  Romania
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Well, I know I am a little late on this but this is due to some problems that kept me away from diy.

Kevin and Chris, thanks for the reply. I've suspected the output cap would be the problem as I allready added a 1uF. Of course the calculations Chris provided made it all clear.
The problem is that for now I won't be able to continue the experiments so at least I'll stick with the theoretic side.

So, could someone help me with getting the source resistance of a srpp circuit? I couldn't locate some reference materials on the net...

Also if someone could direct me to a reference source in designing tube audio would be great.
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