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Old 27th April 2007, 09:33 AM   #1
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Default Memory Distortion? and some new beginnings.

Having built two new amplifier modules that originally started out as doug selfs blameless have gradually grown into something that has created some distance from its predecessor. I have to say, I am very excited abut the Current Feedback pair differential as described in the memory distortion articles, I have built it using 2n5551/5401 biased to 4.5ma tail current. It sounds quite neutral, wonderful, lots of seperation and lucidity. Now I am just curious, has anyone else put this configuration to test?. I had to make some compensation changes, upping the lag comp to 30pf and lead to 5pf to maintain stability. drivers were changed from mje15032/33 to 2sc4793/2sa1837..Any opinions on tail current is 4.5ma neccessary or overkill?.


Colin
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Old 27th April 2007, 09:44 AM   #2
Nordic is offline Nordic  South Africa
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Thank you Colin for restarting this thread... I have been haveing great fun with simulations etc since someone posted those Perrot links....

It struck me how much attention people spend on avoiding distortion mechanisms, yet when it comes to heat induced distortion, they are happy slapping a heatsink on and congratulateing themselves for not reaching thermal meltdown....

Yet they pay attantion to even RF from bridge diodes etc...
However these heated components present a small voltage while cooling down and if I understand correctly useing ohms law, when this meets a high impendance stage you are sitting with a potentialy nasty distortion mechanism, unless you swamp it with feedback...
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Old 27th April 2007, 10:05 AM   #3
AKSA is offline AKSA  Australia
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Default Early Effect

Hi Colin, Nordic,

Let's see if we can avoid another blistering row in this thread!!

I'm not so sure the distortion is all thermal. It's perhaps more related to variation of base/emitter voltage with collector current.

In a conventional bipolar transistor, as you increase collector current the base emitter voltage increase, non-linearly, according to Early effect. This is pronounced with long tailed pairs, and leads to an S shaped transfer function, not unlike the BH curve which defines the magnetisation flux of a transformer.

This S shaped curve means that transconductance of a LTP steadily reduces as an increasing differential voltage is applied to both inputs. The differential input to a power amp is higher than one thinks; if the feedback factor of an amp is 1000, for example (60dB of feedback), then the signal differential between input and feedback nodes is 80mV at 100W/8R output (80Vpp). This transconductance compression is of course dealt with by the feedback network, but it is not eliminated, only reduced.

If you can run the LTP devices in constant current, the Early effect is eliminated, and the transfer function is essentially linear, viz straight.

Colin has already noticed the sonic differences......

Cheers,

Hugh
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Old 27th April 2007, 10:12 AM   #4
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Default Re: Early Effect

Quote:
Originally posted by AKSA
If you can run the LTP devices in constant current, the Early effect is eliminated, and the transfer function is essentially linear, viz straight.
Cheers,

Hugh
Hello Hugh,

Running LTP in constant current means replacing the emitter resistor with Constant current source......results in linear transfer curve then did you use resistor in your amps...

Plz correct me if I were wrong...

regards,
Kanwar
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Old 27th April 2007, 10:28 AM   #5
AKSA is offline AKSA  Australia
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Hi Kanwar,

Send me your email via PM, glad to share......

Hugh
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Old 27th April 2007, 10:36 AM   #6
Nordic is offline Nordic  South Africa
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How do you feel about the writer's opinion, that constant power. i.e. constant voltage AND constant current, would lead to even lower thermal load...

I think some of the claimed improvements are that the peak temperature produced by any one cycle of the signal is shared between more devices, leading to A smaller increase in temp per device, leading to quicker return to ambient temperature...

Once a peak is delivered the device is heated... the problem is that long after the signal passed, the device is not at the same thermal point it was just before the signal reached it... Like you can quickly boil a kettle of water but it will take much longer for it to cool back to ambient if just left to stand...
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Old 27th April 2007, 11:00 AM   #7
AKSA is offline AKSA  Australia
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Nordic,

Constant power is a great idea.

However, consider the electrical environment. If we can achieve constant current, just how much does voltage vary anyway?

Assume a 36V rail, and a 2Vpp signal. This means power might vary from (37 x 1mA) to (35 x 1mA), that is 37mW down to 35mW, with a mean around 36mW. This is plus and minus 2.78%. This is not a large variation, and will not, in my view, result in much variation of temperature on the semi die, particularly at frequencies over 50Hz.

Cheers,

Hugh
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Old 27th April 2007, 11:04 AM   #8
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Quote:
Originally posted by AKSA
Hi Kanwar,

Send me your email via PM, glad to share......

Hugh
Hello Hugh,
You haven't enabled your PM option...

My email is

info@workhorsetechnologiesindia.com
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Old 27th April 2007, 11:26 AM   #9
Nordic is offline Nordic  South Africa
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http://peufeu.free.fr/audio/memory/memory-1-theory.html

Quote:
The black curves show the drift of input pair error voltage Vb(Q3)-Vb(Q4) ; the bottom one is a closeup :

At T=0, VError = 4.98mV
During the impulse, for an output of 30V, VError = 9.302mV (This gives a rather lowish openloop gain of 6940)
Immediately after the transient, VError = 5.177mV
The thermal drifts therefore caused VError to shift of 197uV (this corresponds to the 1.37 value at the output if we multiply it by the OL gain). If we compare this to the VError during the impulse (4.322mV), we come to this shocking conclusion :

The thermal drift signal is only 27dB down from the musical signal !
other pages
http://peufeu.free.fr/audio/memory/

Click the image to open in full size.

Quote:
The memory in the buffer transistor is quite negligible compared to the VAS transistor. Imagine the VAS is confronted to a 50k load (base resistance of the buffer), with a 10mA current sink. Heating the VAS transistor by one degree C will vary its Beta by 3%, which means a drift of 10mA * 3% * 50k = 15 volts that will have to be corrected by feedback. Heating the buffer by 1įC will only make it drift 2mV... The essence of the thing is that memory is multiplied by the gain of the stages downstream ; the VAS has a lot of gain, the follower has none.

If you use an emitter follower for amplification (with for instance 10 Ohm from emitter to ground and 1k from collector to V+) as is done in an input diff pair, you're into trouble, because the Vbe variations are amplified by the gain of the stage. They have to be compared to the input signal, not the output signal.

With a buffer and a small output stage, it is actually an opamp, even if a complicated one... It could be used for anything actually.
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Old 27th April 2007, 11:58 AM   #10
anli is offline anli  Russian Federation
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To eliminate power dissipation deviation (and, as a result, memory distortions of the schematics part which is beyond NFB loop), I use such trick: http://gaydenko.com/phones/schematics41-FINAL.png It is phones "blameless" amp (follower). See Q5 stage: Q6 makes Q5 Vce constant, Q7 makes Q5 Ic constant. AL307Y is a red led (~1.6V drop).
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