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Old 12th March 2006, 09:40 AM   #1
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Default LC AUDIO MILLENNIUM XP - HELP if possible from Lars Clause!

Hi!
I just received (bought) a complete kit of Millennium Xp amplifier from Lc-Audio. But... I can't use it...

I discovered, as you can easily see from the joint schematic, that the input impedance is 2K !! Yes, 2 KOhm!!

Now, I have a simple valve preamplifier, SRPP with a coupling capacitor of 0,47 uF..... I have big problems:
first because the gain of this stage has hardly decreased from this low resistence,
secondly because It cuts all the slow frequency!
I calculated, I need at least a coupling capacitor of 20uF (!)
Too big, to expensive a good one, and too poor will be the sound with this big capacitor.


Now, my questions:

1. Is it possible that the big (really) Lars Clausen, has made a circuit of a PWA with only 2 KOhm input Z? No Power Amplifier has so low input impedance...

2. According to who is so kind to help me, is it possible to increase this input resistance up to, for example, 20-50 KOhm? What will the consequences be?

3. You know what is the gain of the VAS? Because LC audio didn't decleare it on their "cook book"...

4. Any other suggestion?

Thank you very very much in advance.

P.S. I already send TWO e-mail to LC-Audio, of course, but after 10 days I didn't receive ANY answer. It's odd, but It is.
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Old 12th March 2006, 11:25 AM   #2
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It looks to me that you can change this value to any reasonable you want.
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Old 12th March 2006, 12:37 PM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
you should have expected that order of input impedance.

It is a balanced input stage typically driven by output stages that can rise to the demands of 600ohm load impedance. Most are in the range 10r to 50r.

As dark says, raise the input resistors but accept the extra noise.

Regarding the DC blocking cap in your preamp;-
try using a 100uF low esr Electro // 4u7F polypropylene, all times two for your balanced output. Or eight 4u7F polypropylenes giving a high pass of 4.2Hz (F-3), 8Hz(F-1).

How has your pre-amp organised the balanced source to match your new loading?

Can your pre-amp current ability/output impedance drive very long cables?
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regards Andrew T.
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Old 12th March 2006, 12:38 PM   #4
djk is offline djk
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The lower the input impedance, the quieter the amp will be.

Use a 10K:10K input transformer and change the resistors to 10K.

A good input transformer is an investment, not an expense. You can keep it for use with future purchases.
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Old 12th March 2006, 02:38 PM   #5
MiiB is offline MiiB  Denmark
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I can't see you'll run into any significan noise problems by changing to something like 47 Kohm resistors. resistor noise cant be an issue at linelevels

I suspect you are running the tube amplifier singleended so the gnd and ind- are shorted ??

The ultimative problem could be that your tube preamp might also see the 47 Kohm as a hard load. Unless of couse you are using anodefollower output or a supertriode like 6H30.

Best of luck

Michael
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Old 12th March 2006, 08:27 PM   #6
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Voltage gain of VAS is about 60.
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Old 12th March 2006, 08:44 PM   #7
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Default Re: LC AUDIO MILLENNIUM XP - HELP if possible from Lars Clause!

Quote:
Originally posted by andythefirst
Hi!
I discovered, as you can easily see from the joint schematic, that the input impedance is 2K !! Yes, 2 KOhm!!

__SNIP some questions__

P.S. I already send TWO e-mail to LC-Audio, of course, but after 10 days I didn't receive ANY answer. It's odd, but It is.

Hello World!!

- Lower Impedance, the more control is needed from the preamp, the better possibiliti'es for optimal control and sound quality... - And who in the world would use such antiquated things as tubes for other stuff than putting in museums???

There is no problem in changing the input Z resistor to any wanted value. Please keep it under 100K though, and PLEASE do not let the input wander off uncontrolled ( Keep something shielded lownoise attached to the input, no "floating" with high / very high impedances.

The VAS is answered already, no point in repeating that...

Regarding Emails to LC Audio..: - You are probably using the wrong Email Address - where / who did you write??... Try writing to support@lcaudio.dk , and i will answer your qquestion. Only in weekends does it appear that mail grow older than 48 hours ( And maybe if the questions is _very_ weird - but then they are replied by "i'll investigate that - please wait" )...

Sincerely, Lars / Viborg - Tech Support for LC Audio at support@lcaudio.dk
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Old 12th March 2006, 09:14 PM   #8
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Andy,

The reason for low IP reference resistor is mainly OP voltage offset.

For a power amp you have to assume it will be driven by a source
with a cap coupled OP with a relatively high value ground resistor
after the cap.

As such, the IP bias current from the Millenium's BJT stage will cause
an offset voltage. Even though the IP stage is complimentary and
the bias currents theoretically cancel there will be some mismatch
in components and thermal drift which can cause some IP offset. If
the base to ground IP resistor is large, say 50k, the offset can be
considerable.

If the Milleniums servo opamp has say 14V of OP swing, then it can
effectively provide 15V/(301k+499k)= about 18 micro amps of
correction max. This servo feeds the voltage stage which has a
30k load so that 18 micro amps will result in about 0.5V of correction

So if the amps gain is 60 then the tolerable IP offset is less than
10mV for the servo to be able to correct the OP.

The IP to gnd resistor has -nothing- to do with the noise of the
amp.

Cheers,

Terry
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Old 12th March 2006, 09:38 PM   #9
djk is offline djk
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"The IP to gnd resistor has -nothing- to do with the noise of the
amp. "

Then I must be deaf, and RCA must have made up the data in their SSD204 amplifier handbook showing a 16dB improvement in S/N ratio when going from an 18K input impedance to 2K.

Tube guys must be daft too for buying expensive grid chokes instead of just using a resistor.
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Old 13th March 2006, 01:10 AM   #10
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Quote:
Originally posted by djk
"The IP to gnd resistor has -nothing- to do with the noise of the
amp. "

Then I must be deaf, and RCA must have made up the data in their SSD204 amplifier handbook showing a 16dB improvement in S/N ratio when going from an 18K input impedance to 2K.

Tube guys must be daft too for buying expensive grid chokes instead of just using a resistor.
djk,

Relax, and do some more noise theory study.

Any decent preamp should have an OP Z of <1k, usually it is in
the range of a few hundred ohms.

As such the pre's OP Z dominates and the noise produced by
the amps IP to gnd R is shunted by the pre because they are
effectively in parallel.

Maybe you are thinking of a case where the amps noise is
measured without a pre amp connected and without the IP's
shorted. This is in fact a meaningless noise specification because
it does not relate to a real world condition.

WRT your comments on grid chokes, this is a separate and
unrelated issue. The grid choke is in -series- with the IP and
-does- contribute noise separate from whatever pre is driving it.

Also, we are not discussing any sonic effect that such series
resistors may have. We are discussing the noise contribution of
the resistor from base to ground.

Cheers,

Terry
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