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Gain resistors on JLH phono
Gain resistors on JLH phono
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Old 26th August 2018, 01:10 PM   #11
epicyclic is offline epicyclic  United Kingdom
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I have been poring over the gain structure of this RIAA pre amp this morning (I must get a life) and I am confident that I have found a typing error on the below schematic .


On the bottom left it states mm sensitivity of 37mV this should read 3.7mV.


If you look at the bottom right it says the mm gain is 134 which is correct 500mv/134 =3.73mV.


Gain structure... first stage mm 18.16dB


.......................first stage mc 39.33dB


.......................second stage (RIAA) 24dB @ 1Khz
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File Type: jpg JLH phono amp.jpg (95.0 KB, 152 views)

Last edited by epicyclic; 26th August 2018 at 01:16 PM.
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Old 26th August 2018, 02:01 PM   #12
J247W is offline J247W  United Kingdom
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Yes, I believe there has been many typing errors with JLH circuit diagrams, when I first calculated the mm gain using the following information 12mv (at MM) I used the formula 2v / 12mv = 166 & the 166 is the gain which seems to be over the spec written on the circuit ?
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Old 27th August 2018, 12:23 AM   #13
PRR is offline PRR  United States
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> 37mV this should read 3.7mV.

I also doubt the "340pV (mc)"; 340 uV (micro) is more likely.
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Old 27th August 2018, 09:54 AM   #14
epicyclic is offline epicyclic  United Kingdom
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Quote:
Originally Posted by PRR View Post

I also doubt the "340pV (mc)"; 340 uV (micro) is more likely.




Yes agreed .................. I missed that . My excuse is I have been concentrating on the bogus 37mV mm sensitivity .


mc ... 340uV....... 0.000340x1460=0.496V

Last edited by epicyclic; 27th August 2018 at 09:59 AM.
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Old 17th September 2018, 07:06 PM   #15
J247W is offline J247W  United Kingdom
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As in photos you will see the rebuild complete.

The only problem with this RIAA Pre now is the background noise which may be caused by the following information:

Left Channel Input Voltage (not connected to r/p leads) -127mv
Right Channel Input Voltage (not connected to r/p leads) -281mv

With my prototype op-amp based riaa I get a high gain hiss if you turn the volume up like all other riaa pre's I have had well below groove noise.
Now with the JLH I get a deep humming sound, below groove noise but higher than the prototype op-amp based one but still lower than the groove noise, you can feel the hum on headphones if you know what I mean.

Please no fixing advise on earthing as earthing is perfect with no noise.

Could the dc offsets on the inputs be causing this deep rumbling noise & what silicon should I change everything or just the bd's ?

Regards

J247W
Attached Images
File Type: jpg JLH RIAA PreAmp 001.jpg (528.2 KB, 96 views)
File Type: jpg JLH RIAA PreAmp 002.jpg (510.8 KB, 106 views)
File Type: jpg JLH RIAA PreAmp 003.jpg (736.0 KB, 97 views)
File Type: jpg JLH RIAA PreAmp 004.jpg (639.9 KB, 97 views)
File Type: jpg JLH RIAA PreAmp 005.jpg (434.4 KB, 34 views)
File Type: jpg JLH RIAA PreAmp 006.jpg (640.5 KB, 23 views)
File Type: jpg JLH RIAA PreAmp 007.jpg (692.9 KB, 27 views)
File Type: jpg JLH RIAA PreAmp 008.jpg (373.1 KB, 17 views)
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Old 18th September 2018, 10:21 AM   #16
epicyclic is offline epicyclic  United Kingdom
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Your hum problem is probably down to the removal of both 5V regulators , they were there to increase the smoothing on the rails that feed the more sensitive input circuitry. Those voltages at the inputs are about correct , personally I would not use the direct coupled input option .
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Old 18th September 2018, 11:16 AM   #17
J247W is offline J247W  United Kingdom
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Originally Posted by epicyclic View Post
Your hum problem is probably down to the removal of both 5V regulators , they were there to increase the smoothing on the rails that feed the more sensitive input circuitry. Those voltages at the inputs are about correct , personally I would not use the direct coupled input option .
I have been making regulated power supplys for many years so I doubt your answer about the "more precise" +-5Vdc supply rails as they are symetrical voltages being given to the circuit (+-15v & +-5v) not just a "thrown together" Hart psu with way out "plus" to "minus" voltage of up to 2vdc differences


I was thinking the fault is more to do with the ac offsets at the input making the signal to noise hum instead of hiss, my prototype op-amp version is only giving -15mv on each input measured (Fluke MM).

On the other hand a JLH pre-amp is not all it is cracked up to be (people tend to go on about them as "all singing & all dancing") but I have heard Linn systems, "so called" high end equipment & the price they pay does not warrant the hefty price tag & the circuits inside are not different to anything you have seen.

Maybee I expected too much from this design ?
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Old 18th September 2018, 01:53 PM   #18
jamesfeline is offline jamesfeline  United Kingdom
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Hello

The 5v regulators are there for a reason .. also just for info my JLH phono preamp with the Hart PSU does not hum at works faultlessly, as I have a original unit.

You need to retry with the 5V regulators for the front end .. the fact that mine works and yours has a issue there must be a mistake at some area in your work .. just my humble opinion of course .. post up some more pics of your power supply that you have used for a second opinion

Also note the Hart supply has a good star grounding pcb tracks and a good earth lift circuit that actually works ..

all the best
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Old 18th September 2018, 03:21 PM   #19
jamesfeline is offline jamesfeline  United Kingdom
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Hello again

just a couple of pointers

1. is that case earthed ? as it should be earthed to the star point of your psu or a earth lift resistor

2. You have the power supply cables running over the most sensitive part of the circuit, these should be screened or twisted together

3. The 5 v regulators should be installed

4. This design was designed by one of the most respected designers in the field at the time .. I would cheekily say you expect to much from your construction skills not the design
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Old 18th September 2018, 04:22 PM   #20
J247W is offline J247W  United Kingdom
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Originally Posted by jamesfeline View Post
Hello again

just a couple of pointers

1. is that case earthed ? as it should be earthed to the star point of your psu or a earth lift resistor

2. You have the power supply cables running over the most sensitive part of the circuit, these should be screened or twisted together

3. The 5 v regulators should be installed

4. This design was designed by one of the most respected designers in the field at the time .. I would cheekily say you expect to much from your construction skills not the design
After point 4 how do you expect anyone to listen to what you have to say again but on that note I guess your just a Spice Software user who came straight out of school & has no experience in this field.
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