Varying Bass With Different Class-A Amps

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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.
 
AndrewT said:
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.
 
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
 
AndrewT said:
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??
 
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.
 
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.
 
PMA said:
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 ... ;)


I can add my support to this comment. I heard a clear difference in bass performance between my TGM1 and TGM2 yet the corner frequencies were designed as the same in both cases. The main difference between them was that the better bass came from using CFP in the front-end LTP.

Shaan, One thing you have to be careful of - listening at the same volume levels when comparing amps. Higher volume usually sounds 'better' in such tests.

EDIT: I also think that bass sounds 'better' with less H2 distortion (damping factor?), this may be more relevant to these simple ClassA amps. Nice trebles is because H2 distortion takes the 'edge' away, but H2 softens bass.
 
Hello everyone and Cheers on th 10TH ANNIVERSARY of DiyAudio!:)

now,
I just replaced the 220uF cap with 1000uF and there indeed was an improvement in bass... but, somehow, it's powerful but kinda empty, or by the audiophile standards-flabby... what can be the reason?
Plus I am curious about another thing... the DoZ article says the lower BD139 modulates the current between the upper and lower output devices... does it mean that the current flowing through them is not constant(or almost constant)?
And is this a true single ended class-a amplifier?

Thanks for your responses!

shaan
 
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.

I had similar observations comparing Zen4 and Zen9 to LM3886 based amp. For myself I explaned it by the effect of backward signal (emf) from bass driver. One or two stage schematics fight against it worse than the internally multystage LM3886. This effect makes the bass reproduction dependent also on preamp output properties. It seems that preamp faces complex input impedance (with essential reactive components) of amp+speaker system just due to bass driver emf. The smaller and simpler amp schematic, the stronger effect. I got excellent results with powerful preamp based on a SIT transistor (with triode-like output characteristics) + Zen9_50W. I report this in "Crazy" power for preamp" thread that I opened at Upgrade Island, it should be imported to main forums in few days. This combination does not give any chances for LM3886 for being comparable at all. My speakers are Paradigm Studio 100 v.4.
 
[...] I explaned it by the effect of backward signal (emf) from bass driver. One or two stage schematics fight against it worse than the internally multystage LM3886. [...] The smaller and simpler amp schematic, the stronger effect.

Bear in mind that it was reported that the Doz amp had poor bass and it is a multi-stage amp with a substantial output section. Perhaps the issue is not the number and strength of the stages of the amplifier. I suspect it could be related to the feedback design. As I said earlier in this post, I heard a distinct improvement in bass when I changed my TGM1 design into TGM2 by making the input differential LTP devices into CFP's (complementary feedback pairs). The reason being that it improved the linearity and hence accuracy of the LTP which is the primary 'error' amplifier receiving the feedback from the output. In DOZ the feedback is applied to the emitter of the input device. It's a very simple 'error' amplifier and this in my view is the heart of the bass deficiency reported.

If this theory is correct, changing capacitors, improving the power supply therefore would not help. You need to upgrade the front-end of the amplifier to improve the bass. With Nelson's Zen you have an even more basic feedback (via R8) 'error' amplifier.

Pavel's circuit (if it's the one on Rod's web site) is also very simple, yet it has reportedly good bass response. But as I understand it, it's a power buffer and should be used with a pre-amp. Rod suggests using Doz for this. As such, it would provide a nice buffer between the speaker load and Doz.
 
I did drive the MPF by DoZ preamp. It looks like the preamp uses global negative feedback, however, it's not taken from the MOSFET output... the follower probably has some local feedback by the means of TIP142 darlington... I'm not sure... please help me with it.

Bigun, would you please suggest possible mods on the output of DoZ Amp for obtaining good bass? Is it a damping factor problem? Is it the effect of the varying current flowing through the TIP3055s? Or is it the effect of the amp having "Death" in its name? :p :D
 
I did drive the MPF by DoZ preamp. It looks like the preamp uses global negative feedback, however, it's not taken from the MOSFET output... the follower probably has some local feedback by the means of TIP142 darlington... I'm not sure... please help me with it.

Bigun, would you please suggest possible mods on the output of DoZ Amp for obtaining good bass? Is it a damping factor problem? Is it the effect of the varying current flowing through the TIP3055s? Or is it the effect of the amp having "Death" in its name? :p :D

I'm not sure that there's anything wrong with the output of the DoZ Amp. If it is in fact back emf from the speaker entering the feedback loop then the issue is at the front end. The trouble is, damping factor is a bit of an oversimplification - as I understand, the idea is that feedback improves damping factor because it feeds back the 'error' created by the back emf from the speaker and promptly corrects it. But this only works if the back emf arrives at the front end 'error amplifier' with the right phase. I've no idea if this is the case with your set up, if your speakers have cross-over networks inside them the phase could be shifted enough to effectively reduce the damping factor. This effect will be different between the amplifier combinations you have tried.


Do you have headroom to increase the current flow through the output stage ?

p.s. I do not have the experience to solve this problem for you, having built only TGM1 and TGM2 so far (Class AB) - I'm making my best guess though !
 
Shaan: Can you measure or calculate the damping factors for the three amplifiers you are comparing so we can see if there is a correlation with the bass effects you're hearing?

I'm running the amps on an 8ohm fullrange, and has made the amps exactly "as is" except the 1000uF instead of 220uf in the feedback loop of the DoZ amp. So i think the output impedances of the amps I made is close to the published measured value. Well I couldnot find any proper indication of the output impedance of Zenv4 and am too lazy to calculate it as I've abandoned it for having not good enough bass.
The damping factor of the amps is as follows-

DoZ-
Zout=0.378ohm; DF= 8/0.378=21

MPF-
Zout=0.16ohm; DF= 8/0.16=50



...
 
I'm not sure that there's anything wrong with the output of the DoZ Amp. If it is in fact back emf from the speaker entering the feedback loop then the issue is at the front end. The trouble is, damping factor is a bit of an oversimplification - as I understand, the idea is that feedback improves damping factor because it feeds back the 'error' created by the back emf from the speaker and promptly corrects it. But this only works if the back emf arrives at the front end 'error amplifier' with the right phase. I've no idea if this is the case with your set up, if your speakers have cross-over networks inside them the phase could be shifted enough to effectively reduce the damping factor. This effect will be different between the amplifier combinations you have tried.


Do you have headroom to increase the current flow through the output stage ?

p.s. I do not have the experience to solve this problem for you, having built only TGM1 and TGM2 so far (Class AB) - I'm making my best guess though !

Thanks for the info.

I ran the DoZ at 1.7 amps as said... then increased the current to 2 amps and then 2.7 amps... it was hot as hell, but did not show any improvement.
 
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