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Old 4th August 2009, 11:52 AM   #1
shaan is offline shaan  India
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Default Varying Bass With Different Class-A Amps

Hello!

I've been searching for good class-A amplifiers for some time and found Nelson Pass' Zen, Pavel Macura's MPF and Rod Eliot's DoZ as the most popular three. I started with Zen and it really paid me well with it's apealing sound. Then I tried the MPF and found out that the bass is way better than Zen; the mid range and the highs felt same. Then I made the Doz and it gave me good mid and highs, but the bass was worse than the Zen! I made the DoZ exactly as displayed in the original article. I set the bias current of 2Amp for all three amps and the same capacitance multiplier 40V supply.

Can anybody please tell me the reason behind the varying bass response of the three amps? Especially why it is so bad with Doz...

Thanks.
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Old 4th August 2009, 12:16 PM   #2
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Default After 48 years searching for the best amplifier i know the answer



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then write to:

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Old 4th August 2009, 12:19 PM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
are you using Fig5?
C2 looks too small. To match C1, it needs to >=640uF try 680uF or even 1000uF.
Note that fig4 has C2=470uF (for which he states bandwidth) and even that is too small.
What value of C3 have you used?
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Old 4th August 2009, 12:30 PM   #4
shaan is offline shaan  India
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Quote:
Originally posted by AndrewT
Hi,
are you using Fig5?
C2 looks too small. To match C1, it needs to >=640uF try 680uF or even 1000uF.
Note that fig4 has C2=470uF (for which he states bandwidth) and even that is too small.
What value of C3 have you used?

Hi Andrew.

Yes I'm using fig-5 and am using 220uF as C2 in DoZ and 4700uf for output(c3) coupling for all three amps. I too thought of C1 being too small and chose 4.7uF istead.
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Old 4th August 2009, 12:48 PM   #5
AndrewT is offline AndrewT  Scotland
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C1=1uF looks about right with F-1dB~3Hz.
Changing C1 to 4.7uF and leaving C2=220uF is completely wrong.
The amp will develop AC voltage across C2. that may explain why the bass sounds wrong/lacking/distorted.

Try changing C1 back from 4u7F to 1uF and increase C2 from 220uF to 1000uF//1uF
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Old 4th August 2009, 12:59 PM   #6
PMA is offline PMA  Europe
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Subjective perception of bass is usually not much a question of low frequency corner, you may have DC - 50kHz amplifiers still sounding different in low frequencies. It is quite complex question, and I am afraid we still do not have right answers ...
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Old 4th August 2009, 12:59 PM   #7
shaan is offline shaan  India
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Quote:
Originally posted by AndrewT
C1=1uF looks about right with F-1dB~3Hz.
Changing C1 to 4.7uF and leaving C2=220uF is completely wrong.
The amp will develop AC voltage across C2. that may explain why the bass sounds wrong/lacking/distorted.

Try changing C1 back from 4u7F to 1uF and increase C2 from 220uF to 1000uF//1uF

Hi Andrew.

Thanks for the suggestion. So the deficiency is in the design.
Anyway I'm gonna try it with 1000uF as C2.

With the original component values the article says the following

Supply Voltage 40V
Quiescent Current 1.7A
Maximum power 8 Ohms 20W (15W)
Output Noise (unweighted) <1 mV
Distortion @ 1kHz, 15W < 0.2%
Output Impedance 0.378 Ohm
Frequency Response (-0.5dB @ 1W) <10Hz to >50kHz

What's wrong with the last performance measure??
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Old 4th August 2009, 01:02 PM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
try changing C2 to 1000uF and listen,
then try changing C1 to 1uF and listen.
Is there any difference?

The 470uF and the 1uF are defining the low frequency bandwidth.
That guarantees some AC voltage across the NFB capacitor.
The input filter must set the bandwidth, not the NFB loop.
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Old 4th August 2009, 01:34 PM   #9
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Are all three amps using similar power supplies and identical brands of capacitors?

Similar output impedance?



Quote:
Originally posted by PMA
Subjective perception of bass is usually not much a question of low frequency corner

My opinion as well. Sometimes the perception of bass depends on the quality of reproduction of much higher frequencies.
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Old 4th August 2009, 02:18 PM   #10
shaan is offline shaan  India
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Well I used the first track of Pink Floyd's Dark Side Of The Moon and watch the bass behaviour in a "no treable" environment... there are only heartbeats in the first 1 minute and they sound pretty "bassful" in my gainclones. So i thought they would be good for these amps too.
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