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PostPosted: 03 Sep 2015, 19:40 
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Nice clean build. Looks good too.

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Projects: "Carbon” - MM phono preamp | 180W RAW Class D amp | "Illusion" - 6SN7 SRPP preamp in a plastic jiffy box | “Salt Cellar” - Mono 807 triode integrated amp |


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PostPosted: 06 Sep 2015, 05:11 
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Thanks, & does sound good too.
One small issue though - it really has too much gain in my system! A quarter turn of the volume pot with a 12AU7 is way plenty, & about half that with a 12AX7.
Appreciate suggestions on the best way to pad the volume in this circuit to get more useable range on the pot - resistor value changes, resistors on the input/output?


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PostPosted: 06 Sep 2015, 19:14 
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Chris65 wrote:
Thanks, & does sound good too.
One small issue though - it really has too much gain in my system! A quarter turn of the volume pot with a 12AU7 is way plenty, & about half that with a 12AX7.
Appreciate suggestions on the best way to pad the volume in this circuit to get more useable range on the pot - resistor value changes, resistors on the input/output?

If I'm using one of my Class D amps, which are all over 100W and lots of gain, I have the same problem. Because I'm using a cheap pot at the first 1/4 of a turn the right channel is louder than the left. It evens out about the middle.

I can only suggest you use a higher resistance pot. If you are using 100K pot try a 250K. That way you should have to turn it up more before it gets loud. Just a guess. Or add some resistors to up the output of the preamp. Try 50K.

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PostPosted: 07 Sep 2015, 07:59 
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The 4S is running into a 4-5W 6A3 SET amp & 100dB speakers, & I used the recommended 250k pot. No channel imbalance with mine, even though those Tocos pots aren't expensive they do seem to be very good.

Been doing some searching - thought about in-line attenuators which would be like a resistor on the output, seems 2 resistors are used, one in-line & another to ground. Which also reminded me of a Shunt volume control like so:
Attachment:
shuntpotvol2.gif


The shunt resistor should be about 90% of pot value to make it work like a normal pot so for 100k pot you will need a 90k resistor.
Also a shunted pot will:
Reduce max volume by about 5db - good
Increase source impedance when turned more than 3/4 - not likely to use that much volume
Have variable input impedance - by how much? not sure if an issue.


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PostPosted: 08 Sep 2015, 12:06 
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mwhouston wrote:
Chris65 wrote:
Thanks, & does sound good too.
One small issue though - it really has too much gain in my system! A quarter turn of the volume pot with a 12AU7 is way plenty, & about half that with a 12AX7.
Appreciate suggestions on the best way to pad the volume in this circuit to get more useable range on the pot - resistor value changes, resistors on the input/output?

If I'm using one of my Class D amps, which are all over 100W and lots of gain, I have the same problem. Because I'm using a cheap pot at the first 1/4 of a turn the right channel is louder than the left. It evens out about the middle.

I can only suggest you use a higher resistance pot. If you are using 100K pot try a 250K. That way you should have to turn it up more before it gets loud. Just a guess. Or add some resistors to up the output of the preamp. Try 50K.


I have read somewhere to just add resistance to the pot you have. I have not tried this but I cannot see how it would not work, at least no harm done if it does not work.


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PostPosted: 08 Sep 2015, 16:24 
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Chris;

This is a simple issue to fix. The volume control on the output works as a variable voltage divider. You just need to change the range over which the potentiometer acts. Here's how you do it.

First, determine how much attenuation is needed. Typical log taper volume pots represent about -20dBv at the half way point. You indicate that about a quarter turn is took much so that's about -30dBv. You want some headroom, so lets shoot for -20dBv. -20dBv = 10^(-20/20) = 0.1 so this means that the existing pot needs to be about 10% of the total. The volume pot on the 4S is 250KΩ so you need a series impedance in line with the input to the volume control of 9*250kΩ or about 2.25MΩ. There is only one problem, this will radically increase the output impedance go the preamp; up to about 225kΩ. This is too much.

The best way to handle this situation is to move the pot to the input of the stage and put the series resistance there. Then the input impedance is high (which you want) and the output impedance is low (which is also what you want). This is actually a simple rewiring to perform but will require a new pot. I would use a 25KΩ audio pot on the front end with a 250kΩ series resistance with the input. Then just replace the 250kΩ pot on the output with a simple resistor to ground.

This results with a -21dBv pad in the system (which with a 12AU7 in the preamp makes the whole thing about unity gain) and gives you a significantly lower output impedance which should also drive most solid state gear as well.

Make sense?

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PostPosted: 08 Sep 2015, 20:02 
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Location: NZ
Thanks for the detailed reply Matt.
So RCA signal in › 250k resistor › 25k pot › 10k grid stopper.
The 250k resistor will attenuate across the whole range of the pot & keep the input impedance high.

Have a 50k pot, I'd use that with a 250k or 220k resistor?
And the resistor to ground on the output, 250kΩ or thereabouts?

Goldpoint have a suggested mod on their site, thoughts on this? http://www.goldpt.com/mods.html

Attachment:
pre_attn1.gif


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PostPosted: 08 Sep 2015, 23:44 
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Chris;

Almost. Drop the grid stopper as it's not required in this configuration. You want to keep the attenuation value so if you have a 50kΩ potentiometer instead of the 25kΩ then up the input series resistance to 500kΩ. This will keep the attenuation at 50/550 = 0.0909 ≈ -21dBv. As for the resistor on the output, yes, use a 250kΩ.

As for the thing you found on the web, ignore it. They're idiots. :down: Yes the provided diagram of the "pre-attenuator" will provide a reduction of signal. However, it will also adversely interact with the stepped attenuator and screwup the transfer function of the control. It's a stupid approach really because with a little simple math you can add a single series resistor to do the job properly. I wish these sites would have an Electrical Engineer take a look at some of the frass they post on their websites before they post it. Another lesson in don't believe everything you read on the internet.

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PostPosted: 09 Sep 2015, 08:03 
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Suncalc wrote:
...I wish these sites would have an Electrical Engineer take a look at some of the frass they post on their websites before they post it. Another lesson in don't believe everything you read on the internet.



Thank our lucky stars, we have you...

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PostPosted: 09 Sep 2015, 12:33 
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Great, thanks Matt. I'll ignore the idiots!
Yes, sorry should have read your first post more closely, pot 10% of total resistance - guess I could play with values a bit if too little/too much attenuation.
I've got the bias resistors at the RCA In jacks, so guess I''l need to move them to be after the pot.
Will let you know how it works out.


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