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Old 7th March 2010, 01:46 AM   #61
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I came across this thread about using the NE5534 in a very simple scheme. It is "again" a power boosted op amp as usual except that the output section uses an intermediate stage of the opamp and bypasses the output stage.
Will It make an audible difference ? Can only find out by building it ! This one is really simple and can possibly be used direct coupled ( if there is no dc at the input) and has very few components. Might be interesting to see how it compares against the JLH circuit.
I'll post results when I get it done.

Forgot to post the link :
http://www.diyaudio.com/forums/headp...ll-ok-amp.html
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Old 9th March 2010, 01:46 PM   #62
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I tried the JLH headphone amps as a preamp. Works very well. Doesn't make the source sound slightly softer like many average preamps do.
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Old 9th March 2010, 02:39 PM   #63
melorin is offline melorin  Hungary
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Quote:
Originally Posted by ashok View Post
I tried the JLH headphone amps as a preamp. Works very well. Doesn't make the source sound slightly softer like many average preamps do.
Doesn't it make some noise? It's output has low impedance but a preamp usually has high impedance at the output. Is it not a problem?
(I look forward to the PCB's )
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Old 9th March 2010, 04:18 PM   #64
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"........ a preamp usually has high impedance at the output....."

I think you mixed it up. Typically a preamp has a high 'input' impedance and low 'output' impedance to enable it to drive the cable and power amp load.
If you wanted a slightly higher output impedance , all you need to do is to add a 100 ohm ( or whatever ohms ) resistor in series with the output .
Cheers.
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Old 9th March 2010, 05:06 PM   #65
melorin is offline melorin  Hungary
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Quote:
Originally Posted by ashok View Post
"........ a preamp usually has high impedance at the output....."

I think you mixed it up. Typically a preamp has a high 'input' impedance and low 'output' impedance to enable it to drive the cable and power amp load.
If you wanted a slightly higher output impedance , all you need to do is to add a 100 ohm ( or whatever ohms ) resistor in series with the output .
Cheers.
I think that a preamp has high output impedance because typically, power amps have high input impedance (10k-100k). Power amps have low impedance output to be able to drive speakers. But now I don't know, you confused me
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Old 9th March 2010, 05:20 PM   #66
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>If you wanted a slightly higher output impedance , all you need to do is to add
> a 100 ohm ( or whatever ohms ) resistor in series with the output .


Just don't try it with negative resistance. In theory, that too can be an amplifier.
But reality is walking a phase shift tightrope with oscillation on either side.

Shockley (four layer diode) tried, and sorta got that scheme to work.
But drove him out of business, or maybe when his engineers all quit...
http://semiconductormuseum.com/Photo...4E30_Page5.htm

Last edited by kenpeter; 9th March 2010 at 05:29 PM.
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Old 10th March 2010, 02:01 AM   #67
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Hell Ken,
You just confused poor Melorin some more !

Melorin. The preamp has a low output impedance so that it can easily drive the power amp which is not always a high impedance ( unlike tube circuits). It can be as low as 1K ohm in some cases and typically 10k to 20 K ohms. It also needs to drive the interconnecting cable which will have capacitance. With the output impedance of the preamp this becomes a high pass filter. So depending on the value of the capacitance and load impedance you can determine the limiting preamp output impedance to ensure no loss of HF . Often you will find that if a preamp has a 100 ohm output impedance , it is due to a resistor put in series with the output by the designer. The preamp circuit having a very low intrinsic output impedance by itself. It also prevents the preamp from facing a direct capacitive load ( of the cable ) which isn't liked by many circuits.

However this does not mean that a high preamp output impedance will not work ! It will act as a signal divider and will reduce the effective signal available to the power amp and the capacitance loading will have to be looked at very closely to ensure that HF roll off is outside the audio band.
I did some tests with just that. A very high output impedance preamp using tubes. Tube preamp circuits can have ( but not always!) much higher output impedance than solid state circuits. Maybe you should look at my thread about the test I made.
High Zout tube preamps with ss power amps.
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Last edited by ashok; 10th March 2010 at 02:06 AM.
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Old 13th March 2010, 01:25 AM   #68
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Default Further listening tests as a pre amp.

Some more listening tests with the JLH headphone amp as a preamp.
I raised the gain from x 3.2 ( +10dB) to x 5.7 ( +15 dB) because on some discs the signal level was quite low and and even turning up the volume to max didn't clip the power amp output.

Well, it sounds very good. My best configuration was a Solen cap at the input and with no output cap. The power amp has a Solen at it's input. Sounds very good ! The single supply version isn't very far behind either. Though it has about +6 V dc at it's output we can remove the output cap if the power amp has an input coupling cap , which my amp has.
I'm still listening to this combination!
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Old 13th March 2010, 04:41 AM   #69
melorin is offline melorin  Hungary
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Quote:
Originally Posted by ashok View Post
Some more listening tests with the JLH headphone amp as a preamp.
I raised the gain from x 3.2 ( +10dB) to x 5.7 ( +15 dB) because on some discs the signal level was quite low and and even turning up the volume to max didn't clip the power amp output.

Well, it sounds very good. My best configuration was a Solen cap at the input and with no output cap. The power amp has a Solen at it's input. Sounds very good ! The single supply version isn't very far behind either. Though it has about +6 V dc at it's output we can remove the output cap if the power amp has an input coupling cap , which my amp has.
I'm still listening to this combination!
How did you rais the gain? I am thinking of easing down (?) the gain so that it can be too loud for me (could be if it worked well ).
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Old 13th March 2010, 05:28 AM   #70
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Alter R13 and R8 from the circuit shown in the post #15.
I had used 3.3K and 1.5K respectively and changed it to 4.7K and 1 K .
To decrease gain you need to increase R8 from 1.5 K to (?) for example 3.3K . In that case the gain will be x 2 = 1+ R13/R8 . removing R8 will give unity gain. Must also check if the circuit is stable . R13 affects the output offset adjustment and you will need to readjust the preset if it is changed.

I think you should always have an extra + 6 dB or more gain to take into account recordings that are at a lower level. Besides I like the volume to max out much before full rotation !
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Last edited by ashok; 13th March 2010 at 05:30 AM.
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