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preamp volume pot replacement idea


mloutfie

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Recently got a mcintosh c22 diy circuit wit the following schematic. A nice sounding preamp but only have one input and no remote volume. I was going to straight swap a remote 100k volume pot but I found a 10k remote volume pot now I'm thinking to keep the existing pot and just max up the volume and have the remote 10k pot to feed the attenuated signal to it. is that a good idea?

IMG_7355.jpg.ec867055c6784d8943c4057e9b275813.jpg.4981d6c3d679dc9a0c89f4453760124f.jpg

if it's a bad idea and I am putting a remote 100k instead I will need to make this an outboard volume (since there is no space in the case) should I put in 100k loading resistor in the input of the preamp if I remove the existing volume pot? the arrangement would be like this

Source => RCA => 100k ALPS pot with remote => RCA => preamp (with volume control removed)

Obviously the purpose is to remove the cheap volume pot from the circuit. alternatively I can make an umbilical cord that connected between the remote pot and preamp I think this way I don't need any loading resistor in the preamp input. I hope this Frankenstein idea makes sense :)

 

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Bend legs and place one foot in front of the other, problem solved! :ph34r:   :$

 

Edit: I have no idea if good or bad about what you plan, I'm minimalist so view things that way, sorry.

Edited by muon*
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1 hour ago, stereo coffee said:

Hi Mahdi

Change the 100k pot for a fixed resistor,  and use a certain board set   😍 prior to it , with remote volume adjustment. 

 

Cheers /  Chris  

 

I thought about that also but I wasn't sure with the ldr output impedance value. Secondly kinda want different flavours in my system. Using the ldr clearly great sounding. But having a tube is good experience too especially I moved from tube phono to solid state recently. This way I can have 2 preamp.

 

But with your suggestion just put 100kohm both in RCA input leg and the ground leg?

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52 minutes ago, muon* said:

I'm minimalist so view things that way, sorry.

Circuit wise it's as minimal as it get just a pot on the RCA input section. It gets tricky when the case doesn't have enough space for a remote controlled pot and its power board. I guess you can get a bigger chassis but that would be more work

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2 minutes ago, mloutfie said:

I thought about that also but I wasn't sure with the ldr output impedance value. Secondly kinda want different flavours in my system. Using the ldr clearly great sounding. But having a tube is good experience too especially I moved from tube phono to solid state recently. This way I can have 2 preamp.

 

But with your suggestion just put 100kohm both in RCA input leg and the ground leg?

I would listen to it firstly for best audio from the existing pot  , then measure resulting series and shunt resistances , ie input to wiper , wiper to ground.   substitute those values then with fixed resistors.   Failing that try, values of  18k in series and 82k to ground.   

 

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13 minutes ago, Addicted to music said:

there’s heaps of choices, it all depends on what you’re after and question of realestate.   
Definitely ditched any ideas of mechanical pots... that’s for the dark ages.  😂

If my existing 10k motorised Alps pot is not a good idea. My plan is to buy a remote controlled relay switching resistor shunt volume control. With needing to add extra box size won't be an issue. This is the current case there is no room for any kind of input switching or remote board. Did a rough measurement my existing Alps remote would be tough to fit

20210509_115446_HDR.jpg

Edited by mloutfie
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15 minutes ago, stereo coffee said:

I would listen to it firstly for best audio from the existing pot  , then measure resulting series and shunt resistances , ie input to wiper , wiper to ground.   substitute those values then with fixed resistors.   Failing that try, values of  18k in series and 82k to ground.   

 

Thanks will give it a go with the volume pot in first. Might do a try first with the Alps remote on a test case I guess to see how it sounds. I just not sure if the 10k to 100k will have any sort of frequency rolloff. Doesn't have any way to confirm

Edited by mloutfie
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13 minutes ago, mloutfie said:

Thanks will give it a go with the volume pot in first. Might do a try first with the Alps remote on a test case I guess to see how it sounds. I just not sure if the 10k to 100k will have any sort of frequency rolloff. Doesn't have any way to confirm

There is a relationship between the source output impedance and the input impedance, so moving to 10 could have an effect.

Output impedance should be no more than one tenth of the input impedance, or so it goes.

 

Edit: eh...I don't know all the variables at work here so take this post with a pinch of salt :)

Edited by muon*
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Just an update after looking at some input switcher. I found this beauty https://m.aliexpress.com/item/4000775878742.html?spm=a2g0n.productlist.0.0.2cc4721fmZS0qI&browser_id=4111f308829a4d08b865204123174bc7&aff_trace_key=57dc5f04c20f48c29437929b3f3c0d98-1620128053716-09641-UneMJZVf&aff_platform=msite&m_page_id=eepwgfkdfracatg51799163d648f2f53cde2327a6d&gclid=&_imgsrc_=ae01.alicdn.com%2Fkf%2FHaf017272d49b4edc9ec63f960c75749bv.jpg_640x640Q90.jpg_.webp

 

It's remote controlled input selection with a relay based 128 step resistor shunt remote volume. With the price it's a no brainer I was going to spend almost that much on remote input switcher. At the moment I'll play around with the stereo coffee ldr feeding through the existing volume pot

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  • 2 weeks later...

Put a grid leak resistor according to tube spec and fit a Tortuga ePotV3 Mini with LDR module in place of the volume pot.

Small enough to fit inside the enclosure and no need for an external box.

Professional design, quality build and prompt delivery unlike some.

Edited by Ihearmusic
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12 hours ago, mloutfie said:

What is that and for what purpose?

It is there for the electrons that accumulate on the grid the be able to trickle away so to speak. Otherwise the grid goes more and more positive.

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I've been experimenting for more than a week now using an outboard non 100k pot and it is not that straight forward. Impedance matching is definitely a key aspects on the performance.

 

First issue the volume pot definitely control not only attenuation of the source. It also control the gain of the preamp so having the volume wide open is a no go since will really increase noise floor.

 

Second issue driving it with an external passive volume has problem with the volume setting I set (the original plan is to replace the pot with a single resistor in the path). Setting it low will give less noise floor but loose dynamic setting it high will increase dynamic but with higher noise floor.

 

Unless there are suggestions on how to modify this circuit to be just tube buffer. Looks like the best option is putting a new much better 100k passive volume control like the one I linked before. I like the sound of an ldr passive but the impedance looks like not suitable for this circuit

Edited by mloutfie
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  • 3 weeks later...

Tried using this now https://www.ebay.com.au/itm/294193259826?var=592870837994 it's a beautiful step up from cheapo pot $12 I always replace pot with stepped potted but normally around $50-70 per put this is such a bargain. sound wise it's great not sure with longevity of the switch but the build quality and pot feel is great.

1339883106_s-l1600(5).jpg.6421a734699fb77ee3c2694fd27d5533.jpg

Edited by mloutfie
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