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Old 4th March 2003, 03:02 PM   #1
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Default Series vs shunt feedback listening test

I've been meaning to do this for a long time. The basic circuit is two (valve) SRPP stages followed by a source follower MOSFET stage.
1. Voltage series (VSFB)feedback from output to the cathode of the 1st SRPP stage.
2. Voltage shunt (VHFB) feedback in first SRPP stage and voltage shunt feedback from output to input of 2nd SRPP stage. This splits the amp into two sections with no global negative feedback like the voltage series configuration.

The listening test: Transients in the VSFB was better than in VHFB . Much tighter .
The bass was also tighter in VSFB vs VHFB. Deep bass was present but better defined in VSFB.

Voice. Voice was noisy in VSFB as compared to VHFB where the voices were VERY good. Treble was a little laid back in VHFB. The overall sound was much cleaner and sounds spaced apart in VHFB. Bass drum was 'meatier' in VHFB. Traansients were not as dramatic in VHFB.

VHFB was easier on the ear and more musical. Voices were very real and upfront. The speakers kind of disappeared. But VSFB had punch and impact in comparison with VHFB ( good for rock?).

So what do I choose? The VSFB for the bass end (sub) and VHFB for the mids and everything above. I do not mind missing some punch for a more musical sound. VHFB sounds "bassier" than the VSFB. That could be because of less control over the speaker due to higher output impedance at LF.
Anyone out there with a similar or a different experience?
Cheers.
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Old 4th March 2003, 03:10 PM   #2
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Interesting observations. Very similar to my findings with solid state. Never thought NFB was meant for valves though. Did you try no NFB? You probably have one stage too much for that.


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peter
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Old 4th March 2003, 03:25 PM   #3
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Default No Feedback test

Hi Peter,
I did try the no feedback scheme with one input SRPP reduced.
My conclusion is that no NFB is certainly the best sounding for 'voice'. There is no two ways about this. But the high output impedance is a problem as it makes the bass flabby and bloated.
The HF is also nice but does not have as much bite as in Voltage Series feedback. If one had to choose "only one circuit" I would go for Voltage shunt as the voice is pretty close to the no feedback version. If you want to be a purist and do not mind the bass because you have a separate sub , then no feedback for 100Hz and up should be good if not very good.
Cheers,
Ashok.

Voltage Series feedback HAS to be the choice of "rockers". The transient attack and taught deep bass is incredible.
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Old 4th March 2003, 07:08 PM   #4
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Asok,

Good observation. I like Peter have made the same observations on solidstate equipment.

I hope Frank could chime in. He is Mr Tube...........well Frank?


Jam
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Old 5th March 2003, 02:13 PM   #5
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Default ...Interesting...

With solid state Class A amplifiers I ended up with a series regulated supply feeding a shunt regulated supply for best sound.

These amps were IT coupled between input/voltage gain stages and driver/output stages with no global feedback. They were also fully balanced designs.

I've been playing with the two options and combinations with a valve amplifier and haven't reached conclusions yet as my passive supply sounds better than either regulated supply or one reg feeding the other... From which I conclude that I need to do more work on my regulators...

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Old 5th March 2003, 03:38 PM   #6
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Default Long term burn in.

Hi,
I was beginning to think that the first few minutes / tracks of music that we know well are the most important .
Since we have ears that tend to get tuned to the sound if we listen long enough , we could end up getting used to the 'new' sound - good or bad.

So long term burn in may often be 'burn in for our ears'. So a quick test - no more than a couple of hours should tell us if we hear a difference. If we cannot then I think maybe there is no difference at all. The mind is great at playing tricks , especially if we want it to !
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Old 5th March 2003, 08:15 PM   #7
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Default FEEDBACK.

Hi,

Quote:
He is Mr Tube...........well Frank?
Thanks, Jam.I'm sure I am not the only one around here though.

With regard to feedback;I try to avoid global NFB as much as possible and try to make each stage as linear as it needs to be before applying any kind of feedback to it.

As you all know I am pretty much ignorant about semi-conductors but with most triode valves you can use local (degenerative FB) at the expense of Zo.

Not many people are aware of it but a triode has FB built in already and if you operate it in its most linear area you often don't need feedback.
Other techniques such as distortion cancellation techniques can be used.

As James has pointed to regulation of the PSU, this too plays a very important role in the sound of a circuit...especially with the SRPP types which are senstive to the PSU.
Quite likely because the don't really shunt regulate all that well in their standard form.

So, as far I can tell, I wouldn't make general conclusions although I do tend to agree with Ashok's findings, generally speaking.

Cheers,
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Old 5th March 2003, 08:35 PM   #8
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Default Series or shunt feedback!

I prefer the sound of series feedback.!

I have expressed the same feelings not so many time ago in the solid state forum...the shunt feedback sound to may ears much more dull.

The explanation of this is that in the shunt (inverting) configuration the EMF from the speaker find a way via the feedback resistor to the cd or preamp that is feeding it!..that is the cause of that dinamic intermodulation! And the dull sound.!

Regards

Jorge

PS. people when talk about topologies and distortion allways forget that the amp is feeding a reactif load!
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Old 5th March 2003, 09:59 PM   #9
jam is offline jam  United States
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Frank,

I guess you are not going to give up your favourite pre-amp. schematic? Must be some family secret.

I agree with Mr.Pass that keeping the number of stages in a design to a minimum is important. I guess if you keep the feedback path short the negative effects of feedback are less apparent.

Jam
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Old 5th March 2003, 10:20 PM   #10
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Default Preamp.

Hi Jam,

Quote:
guess you are not going to give up your favourite pre-amp. schematic? Must be some family secret.
Italian restaurants don't hand out recipes either...
Still, if I can help...tell me what you need from a preamp: such as do you need phono RIAA correction, line etc...maybe I can help.

It looks as if I can easily persuade you to use tubes then you'd be convering me to the sand camp he?

Quote:
I agree with Mr.Pass that keeping the number of stages in a design to a minimum is important. I guess if you keep the feedback path short the negative effects of feedback are less apparent.
So do I, better to use local loops than global ones if you must use FB.

Cheers,
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