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Old 10th June 2008, 03:31 PM   #81
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by AKSA
Andrew,

You've introduced quite a few other issues into this, but why is my example bad design? I'm curious..... and I do not agree that there are two design camps here, I have use both, but have found the stock CM is flawed for reasons I alluded to earlier.

Hugh
Hi Aksa,
I am saying that Glen would criticise these current values as bad design and his reply virtually said so. I accept your values as examples and they do indeed show that VAS base current forces the mirror to supply unequal currents to the two halves of the LTP.
Even halving the VAS current and doubling the VAS hFE still results in a 2% imbalance in the LTP currents. The mirror will also have an imbalance of around another 0.5%.
Glen obviously is aware of this problem but addresses it by what he calls good design, implying that any other method is bad design.

You are in the resistor loading camp, Glen appears to be arguing that the mirror camp is better.
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Old 10th June 2008, 11:40 PM   #82
AKSA is offline AKSA  Australia
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Hmmm, Andrew, thanks, I can wear that. We should take Glen's comments lightly, he is just trying to educate!

I use LTP resistor loading (with particular attention to LTP balance) on two of my amps; on another three I use a modded CM. Some years ago I had noticed that resistive loading sounded better (when balanced) than CM, but recently discovered a mod to the CM (effectively a Widlar) which restored the sonics and gave the twin benefits of higher fb factor and automatic offset control. This eliminates a trimpot, which saves money and time.

Glen makes the point that charge/discharge of the Miller cap on the VAS is effected alternately from the LTP and the CM. Both circuit paths are collectors; but I would not expect their source impedances to be identical in any event, so slight variations in currents would make little difference except at the very highest frequencies, presumably approaching the HF pole.

While I don't generally condemn topologies as good/bad design, I can see Glen's point; everything is a compromise, and in light of the fact that with some work almost any configuration can be made to function, I try to choose topologies which give the best sonics, at least in my view. All of these circuits are capable of vanishingly low distortion, so rather than agonise over distortions that are 140dB down in sims, I try to road test each new circuit where subjective differences are quite apparent. It is very easy to be seduced by the intellectual purity of a topology, only to discover on building it that it does not sound good.

Focussing on the empirical is time consuming, but I'm more than prepared to listen to the experts here. From time to time, they have some very good ideas. A classic case of brilliant design is, in my view, Mihai Rauta's 'Roender' amp.

Cheers,

Hugh
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Old 11th June 2008, 12:03 AM   #83
GK is offline GK  Australia
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Quote:
Originally posted by AndrewT
Glen obviously is aware of this problem but addresses it by what he calls good design, implying that any other method is bad design.


You brought up the a few issues that effect the current balance of the LTP when using a current mirror, ignoring the fact that without the current mirror the situation is generally far worse. I explained how to address a few of these issues and how not to address them. All you've done since is complain. This is getting boring.
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Old 11th June 2008, 01:32 AM   #84
AKSA is offline AKSA  Australia
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Glen, let it go.

For one who can really dish it out, you are incredibly sensitive. There is just no point to it.
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Old 11th June 2008, 01:58 AM   #85
Lumba Ogir is offline Lumba Ogir  Sweden
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Hugh, how are you?
Glen is a kind person, just like you.

Glen, have you fixed up the garage?
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Old 11th June 2008, 02:32 AM   #86
GK is offline GK  Australia
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Quote:
Originally posted by AKSA
Glen, let it go.

For one who can really dish it out, you are incredibly sensitive. There is just no point to it.

Erm, who is being sensitive? I just summarised the preceding discourse.

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Old 30th August 2008, 10:40 PM   #87
lineup is offline lineup  Sweden
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Quote:
Originally posted by AKSA

The input transistor would have around 24V Vce;
the feedback transistor around 24V6.
This would have no effect on diff action at all.

A current mirror increases OLG very considerably,
increasing fb factor, and thus improving THD, bandwidth and Zout.

The increased feedback factor focusses attention on amp stability;
the OLG must be brought to below unity by the pole frequency,
and this too involves some compromises.

There is no free lunch,
and both approaches can be made to work very well.

About be paranoic obsession to have
the voltage across each LTP pair Transistor absolutely equal.
This is what we often see beginners do.
Not experienced amplifier designers, so much.
The only thing you wull acheive with this
is a tiny bit lower gain, in theinput as a whole.
And this is no improvement. Is it?

(One exception note is in place here.
When I design as low voltage amps as only 2 - 5 Volt supply
I will sometimes try to equal the voltage across the both transistors in diffrential input.
But for voltages above 5 Volt or as much as 20 Volt C-E,
we can forget about robbing one transistor of voltage, for no use.)

Again, AKSA explains one case where is no free lunch.
Using CCS, instead of resistor, will change the overall parameters of the amplifier
at such a degreee that measures may have to be taken.
To avoid instability and misbehavior.

Finally.
Very good amplifier idea, Dxvideo
Thanks for one interesting Topic

Lineup
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Old 31st August 2008, 10:09 AM   #88
homemodder is offline homemodder  Portugal
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Hi Lineup,

Another old topic, but still controversial one. On a subjective note close balancing has the benefit of creating wider more precise soundstage. On a electrical note reduces distortion, both high and low order.

For beginners, I dont think so......., more for advanced design I would say.............
Read Aksa reply......... Using resistive loading with particular attention to balancing........and resistive loading sounded better when balanced......., this should tell you something or are hundreds of peope who listen to his amps wrong about its sound quality.

There is no problem using mirrors either, can sound very good depending on the design of mirror. Ordinary ones sound awful.

Minor compromises such as stability has to be looked at with mirrors, but hey look at all the other benefits, subjective and electrically.
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Old 31st August 2008, 10:22 AM   #89
lineup is offline lineup  Sweden
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Quote:
Originally posted by homemodder
Another old topic, but still controversial one. On a subjective note close balancing has the benefit of creating wider more precise soundstage. On a electrical note reduces distortion, both high and low order.
For beginners, I dont think so......., more for advanced design I would say.............
Read Aksa reply
I do not get you point, homemodder.
Is this a reply to my post .....
What statement in my post you refer to ??
Or was your reply directed to AKSA ???

Regards Lineup
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Old 31st August 2008, 12:35 PM   #90
homemodder is offline homemodder  Portugal
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Hey lineup, was addressed to you, but sorry I misread what you wrote, you talking about VCE balancing, my answer adresses IC balancing.
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