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Old 2nd November 2006, 11:57 AM   #11
mmerig is offline mmerig  United States
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Quote:
here we can download HAFLER pre- and power amplifier service manuals
There is no special manual for your phono amp
but the phono schematic may be in some of these manuals.
Thanks, I was aware of the Hafler downloads, and have the manual for the DH110 (my preamp) and the DH101 (whatthe DH102 was meant for). I asked Hafler if they had a schematic for the DH102, and they do not.

Without the cap and resisitor that I added, the DC offset remains high. but as "lineup" suspected, my measurements were with no cartridge or load attached.
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Old 5th November 2006, 04:25 PM   #12
mmerig is offline mmerig  United States
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The DC offset with 150 ohms across each output to ground is less than 1mv, so the head amplifier should be okay to use.

Thanks for all of the great advice!
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Old 5th November 2006, 04:39 PM   #13
lineup is offline lineup  Sweden
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Quote:
Originally posted by mmerig
The DC offset with 150 ohms across each output to ground is less than 1mv, so the head amplifier should be okay to use.

Thanks for all of the great advice!
good

<1 mV is a bit better, for sure!
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Old 5th November 2006, 04:49 PM   #14
AndrewT is offline AndrewT  Scotland
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Hi,
now that we know it's safe to use,

Quote:
is a bit better
what effect on the symmetry of the stylus will the 1mV of bias have on the motor structure?
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Old 19th November 2006, 05:18 PM   #15
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Quote:
Originally posted by AndrewT

what effect on the symmetry of the stylus will the 1mV of bias have on the motor structure?
Now, that's what I want to know as well. My current diy-phono has about 0,6mV offset, and the carts output is 0.4mV. It does sound better than having another opamp in the first stage that makes 0.0mV, though.



Rüdiger
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Old 24th December 2006, 01:05 AM   #16
mmerig is offline mmerig  United States
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I installed the head amp in my pre-amp, and have been using it for a while now. It sounds fine.

The dc offset with a 20 ohm load (like the cartridge) was less than 0.1 mV, and the current was about 0.5 uA.

There is a 220uf NP capacitor in series at the input, so I would guess that this blocks most if not all of the DC. The original design had a polar cap (220 uf) in series. Besides being about 30 years old, I figured a non polar cap would be better than the polar one.


Thanks for all of the help.
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Old 24th December 2006, 07:40 AM   #17
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A cap coupled phono input is still a sonic disaster. At uV levels electrolytic caps seem to be really non-linear.
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Old 24th December 2006, 08:02 AM   #18
lineup is offline lineup  Sweden
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Quote:
Originally posted by mmerig

There is a 220uf NP capacitor in series at the input,
-----
I figured a non polar cap would be better than the polar one.
Quote:
Originally posted by analog_sa

At uV levels electrolytic caps seem to be really non-linear.
Hello.

Beside less linearity, at those small small levels
the NOISE of an Electrolytic Capacitor could be considerable.
At least this is what I think.

Changing that 220uF Lyt into
- two Paralled 4.7 uF non-polar Film Caps ( = 9.4uF )
- or one 10 uF dito
would in my opinion be a Huge Improvement

Could be polyester MKT or polypropylen MKP filmcaps.

I would not try Ceramics, Tantal or Electrolytics Coupling to any Input Transistor,
if I have one good alternative.

For Output is different. Because here the SIGNAL is often Bigger than a few uV and mV
and so the eventual noise from Cap is a level BELOW signal.


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Old 24th December 2006, 11:02 AM   #19
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Hi,
I have found any cap i had at hand at the input a serious degrader of sound.
Rüdiger
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Old 24th December 2006, 01:38 PM   #20
lineup is offline lineup  Sweden
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Quote:
Originally posted by Onvinyl

I have found any cap i had at hand at the input a serious degrader of sound.
Rüdiger
Yes, this is what we would go for!
And what I know, most modern RIAA Phono amp have no input cap.
This is because they are using dual supply, V+ V- and input at Ground level, 0V.
Only one resistor to ground = input impedance
and one small cap in parrallel with this input resistor
to set the desired capacitance needed for the PickUp in question.

For MM pickup usual values are:
Rin = 47kOhm
C = 0-100pF
( value depends how much capacitance the connection cable contributes
and of the recommendation in instruction manual for the PickUp used )

But here we are dealing with a somewhat older type of RIAA Amplifier.
This is why an input Capacitor is used and Let's keep it like this!
Sure there is a valid reason for it!
The designer is not a schoolboy when comes to circuit designs.
It is one professional designer!


-----------------------------------------------

At the output most every RIAA pickup amplifier needs a Cap (maybe 2.2-4.7uF Film Cap).
Because of the high gain of these amplifiers considerable output DC-offset can occur.
On the other hand,
If your RIAA amp output is feed to a preamp/power with input cap
you should not need any output cap in your RIAA amp.


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