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Old 4th February 2013, 08:01 AM   #1
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Default Improving Bass Response On A Popular Hybrid Design

Hi All,

I just finished building a Sijosae v3.0a amp using 1% metal films resistors and ordinary budget caps. I used the IRF610 and MJE15030 transistors, and a Ruby 12AU7 tube. I'm noticing a sharp decrease in bass reproduction. I don't have any means of recording this, I'm just noticing a decrease in my 32Ohm headphones.

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Which parts would be recommended for replacement in order to make this amp capable of lower frequency reproduction?
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Old 4th February 2013, 11:36 AM   #2
DUG is offline DUG  Canada
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"I'm noticing a sharp decrease in bass reproduction."

Compared to when? or what?

The only caps directly in the signal path are the 1000uF.

Those into (47R+32R) give a 3dB of about 2Hz.

I don't know what the symbol is at the bottom labeled with a 4,5,9.

I hope it is a resistor of some type because the MJE15030 bases are biased at 6.8V.

It looks like it should be a class A amp...

What is the current through the output stage?
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Old 4th February 2013, 12:54 PM   #3
DF96 is offline DF96  England
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4,5,9 are the heater pins on the 12AU7. The valve heater is being used as the emitter resistor for a 'CCS'.

I hope your source has guaranteed low DC offset, because you are connecting it straight to the valve grid.

The "decrease" in bass is with respect to what? Maybe your previous amp had a bass boost, which this one lacks? As DUG says, the electrolytic cap is the only thing limiting bass response and it is plenty big enough.

With such a low supply voltage you can expect some distortion from the valve, but this should not affect bass any more than any other frequency. The cathode pots will eventually become noisy due to the track-slider current, but again this will not affect bass. This design has plenty of flaws, but it should have plenty of bass.
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Old 4th February 2013, 10:50 PM   #4
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DUG: I'm comparing it to the output of the integrated headphone amp inside my computer. I plugged the output of the integrated amp (which has significantly higher bass response) into the input of this amp to test whether the lack of bass was from my source media or the PCM2706 DAC I built. Both cases have diminished bass through the hybrid amp.

The symbol at the bottom is the 2 heaters of the tube going into ground through their center tap.

At which two points would I break the circuit to accurately measure current at the output stage?

DF96: What would a low DC offset be? I'm hoping to use this as the output amp from a PCM2706 DAC chip, as per the following two diagrams (I'm using the output from the schematic for the PCM2704 example).

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Old 5th February 2013, 03:46 AM   #5
DUG is offline DUG  Canada
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Ok, now we can compare an amplifier with a few ohms of source impedance with one of 47 ohms of source impedance...there might be something in that.

If your headphone impedance gets lower with frequency ( a distinct possibility) it will get the same voltage drive out of the computer amp and draw more current and therefore more power.

This amp would provide reduced voltage because of loading and therefore less power at those lower frequencies.

I could see that happening...

Just my theory.

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Old 5th February 2013, 10:31 AM   #6
DF96 is offline DF96  England
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Provided the DAC has an AC coupled output with ground leak resistors, as shown in the two diagrams, you should be OK to connect straight to the valve grid.
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