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PostPosted: 04 Jun 2014, 22:08 
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ewharff wrote:
I am building another one of these amps. I am going to put it into a case and I want a volume control on the case. What pots are you using? I cannot seem to locate a decent 1M stereo pot anywhere. Thanks for the help.


You can use lower value pots e.g. 250K. In the past I have used miniature cct. brd. pots. I can get them in 1M (I think its been a while)

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Projects:“MWH” - 1920s UX-171A preamp | ”Image” - Low Profile 100W Class D amp | ”fiih” - 150W Holton inpsired power amp | "Precision" - 300W Class AB Holton power amp


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PostPosted: 13 Mar 2017, 05:55 
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It's been a while since we have had a post here. My daughter has been studying for uni so I'm on the phones these days. I'm using a Pi2/HDMI audio extractor as my source and DAC and an original K272A as the driver HP amp for my Sennhieser 700s. Originaly my K272A didn't have a volume control so I was using Volumio's software volume control. It sounded really good. I then upload Rune to the same Pi but the software vol. control is rubbish. Volumio's is much better.

To remedy this I have now added a 50K micro PCB pot to my HP amp. The musical is back and I have disabled the software vol. control in Rune. I line my ABS encsloures with bituamised heavy Al foil but not earthed them. As a result when my hand got near the HP amp it hummed. Now earthed and dead quiet.

While doing the above I noticed I had modded the K272A to run 18V on the tubes. Also I noticed I used three-nines 0.7mm pure silver wire as hookup wire. Also I saw all the resistors are carbon film. I haven't looked inside this HP amps for many years. This K272A sounds absolutely fab and I was wondering why. No I know. It is an absolute gem and plays all my 24/192 files from the Pi2B and 2T drive with guts and glory. I hope my daughter never finishes Uni - lol.
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Projects:“MWH” - 1920s UX-171A preamp | ”Image” - Low Profile 100W Class D amp | ”fiih” - 150W Holton inpsired power amp | "Precision" - 300W Class AB Holton power amp


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PostPosted: 17 Mar 2020, 17:11 
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mwhouston wrote:
While doing the above I noticed I had modded the K272A to run 18V on the tubes.

Holy thread revival, Batman!

Out of curiosity, what mods did you perform to have it run 18V on the tubes? Filament voltages? I purchased a kit with upgrade parts from you way back in Feb 2012 to use as a preamp. (with an OPA2134) It has been collecting dust since the purchase of my Yaqin MC30L back in 2015. I've recently decided to pull it back out for HP duty. Since I run Li ion pretty much exclusively these days, I was wondering how to mod mine for 12.6v. (peak surface charge of 3 cells) Have just been running 2 cells currently, but when I tried out my trusty old 32 ohm AKG's, there was considerable distortion. I wrote this off as possibly having to do with the 2134 op amp. Reading further into the voltage upgrades in this thread though, I realize it could be a low voltage situation. Also, if I understand things correctly, low internal cell resistance in this application is a good thing? (on the subject of noise)

Any help would be greatly appreciated!

Thank you in advance!

-Phil


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PostPosted: 17 Mar 2020, 20:52 
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In an attempt to possibly answer some of my own questions, I decided to have a look at the IC datasheets.

It appears that the operating supply voltage range for the L4949 regulator is 5-28 vdc. Well within the range I'm looking for. I presume that the 270K R9 drops the 5V output to approx 2.5V for the filaments in series. So, filaments good to a theoretical regulator input voltage of 28V?

Looks like the other leg of regulated 5V goes to pin 8 of the op amp. So same goes there too? Sounds about right for the PT2308 as datasheet spec is 8V maximum supply voltage. So, okay on the op amp too? I found it a bit interesting that the OPA2134 has a supply rating of up to 18V. (30V @ absolute max)

So, I also presume the 6418 plate voltage is whatever battery voltage available? Minus the 180K R1 and R2? Up to 30V max (also according to datasheet)

If I might be missing something else about the circuit and/or opreating voltage dynamics, could you help a layman?

Again, thank you in advance for helping me along my quest of understanding!

*edit
I believe I've exauhstavely read and filtered through almost every relevant post about all the incarnations of the K272. I'm now very interested in not only having separate filament supplies as was achieved in the "b" model prototype you built, but also a separate regulation method for them if at all possible. I tried to zoom in on the picture you posted of the schematic, but couldn't read the print. Were they dropping the voltage from 9V B+ with a resistor per filament? Values?

Goldkenn had mentioned some modding ideas he had and one of them included the mention of an LM1117 1.2V 1.0A fixed output linear voltage regulator. Datasheet says a maximum supply voltage of 20V. Would grounding (or rather lack of) the cathode present a problem in an isolated PS configuration mod to the K272 version 1?

Apologies for my half versed knowledge of audio amplifier theory.


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PostPosted: 18 Mar 2020, 06:07 
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e_phil_b wrote:
mwhouston wrote:
While doing the above I noticed I had modded the K272A to run 18V on the tubes.

Holy thread revival, Batman!

Out of curiosity, what mods did you perform to have it run 18V on the tubes? Filament voltages? I purchased a kit with upgrade parts from you way back in Feb 2012 to use as a preamp. (with an OPA2134) It has been collecting dust since the purchase of my Yaqin MC30L back in 2015. I've recently decided to pull it back out for HP duty. Since I run Li ion pretty much exclusively these days, I was wondering how to mod mine for 12.6v. (peak surface charge of 3 cells) Have just been running 2 cells currently, but when I tried out my trusty old 32 ohm AKG's, there was considerable distortion. I wrote this off as possibly having to do with the 2134 op amp. Reading further into the voltage upgrades in this thread though, I realize it could be a low voltage situation. Also, if I understand things correctly, low internal cell resistance in this application is a good thing? (on the subject of noise)

Any help would be greatly appreciated!

Thank you in advance!

-Phil

I’m currently building two very big power amps for others; a 300/500W Holton Precision Class AB in a 4U for one a Pass F5 turbo in an U5 for another. One almost complete the other only been working on for a few days. So time is short. Tomorrow I’ll pull out the schematic for the 272 where the OPA2134 runs on 18V and the 6418 on 5V. It requires cutting a track and a link. A very sharp blade is required.

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Projects:“MWH” - 1920s UX-171A preamp | ”Image” - Low Profile 100W Class D amp | ”fiih” - 150W Holton inpsired power amp | "Precision" - 300W Class AB Holton power amp


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PostPosted: 18 Mar 2020, 10:13 
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mwhouston wrote:
e_phil_b wrote:
mwhouston wrote:
While doing the above I noticed I had modded the K272A to run 18V on the tubes.

Holy thread revival, Batman!

Out of curiosity, what mods did you perform to have it run 18V on the tubes? Filament voltages? I purchased a kit with upgrade parts from you way back in Feb 2012 to use as a preamp. (with an OPA2134) It has been collecting dust since the purchase of my Yaqin MC30L back in 2015. I've recently decided to pull it back out for HP duty. Since I run Li ion pretty much exclusively these days, I was wondering how to mod mine for 12.6v. (peak surface charge of 3 cells) Have just been running 2 cells currently, but when I tried out my trusty old 32 ohm AKG's, there was considerable distortion. I wrote this off as possibly having to do with the 2134 op amp. Reading further into the voltage upgrades in this thread though, I realize it could be a low voltage situation. Also, if I understand things correctly, low internal cell resistance in this application is a good thing? (on the subject of noise)

Any help would be greatly appreciated!

Thank you in advance!

-Phil

I’m currently building two very big power amps for others; a 300/500W Holton Precision Class AB in a 4U for one a Pass F5 turbo in an U5 for another. One almost complete the other only been working on for a few days. So time is short. Tomorrow I’ll pull out the schematic for the 272 where the OPA2134 runs on 18V and the 6418 on 5V. It requires cutting a track and a link. A very sharp blade is required.

Thank you for the prompt response, that would be awesome! Sounds like you have a heavy project load for sure. I'm not in a huge rush, so take your time getting back. And heres hoping your projects go smoothly, Cheers!


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PostPosted: 18 Mar 2020, 17:43 
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Cut the track which feeds 5V to the chip which now MUST be an OPA2134 or equivalent. Add a link to the chip V+ (pin 8) from the battery positive rail (now 18V). Ignore all other markings on the image. Only apply the changes which are in the ORANGE circles. No others.

DO NOT APPLY 18V TO THE PT2308 all the smoke WILL come out. See image.
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PostPosted: 19 Mar 2020, 11:17 
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Wow, I like it! Fairly simple change. Do you think the 2134 sounds better with the higher voltage than the 2308 running with a regulated 5V? I remember your previous comments on being partial to the sound of the 2308 for HP duty. Looked up your Sennheiser 700's. I presume these are the HD700 model (what a nice set of cans!) Are these indeed 150 ohms? It says a lot if that configuration is driving them with authority.

Will post pics as I get underway on this project. Was digging through a parts box stashed away and found a treasure trove from forgotten projects gone past that are very will suited for this mod. Found my dip8 2308's and extra 2134's too! (2x each) Also some other opamps I can't remember the # of right now that I was probably going to play with rolling.

Thank you again for taking the time out of your busy schedule to post pics, Mark! Mucho appreciated.

-Phil


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PostPosted: 19 Mar 2020, 15:58 
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e_phil_b wrote:
Wow, I like it! Fairly simple change. Do you think the 2134 sounds better with the higher voltage than the 2308 running with a regulated 5V? I remember your previous comments on being partial to the sound of the 2308 for HP duty. Looked up your Sennheiser 700's. I presume these are the HD700 model (what a nice set of cans!) Are these indeed 150 ohms? It says a lot if that configuration is driving them with authority.

Will post pics as I get underway on this project. Was digging through a parts box stashed away and found a treasure trove from forgotten projects gone past that are very will suited for this mod. Found my dip8 2308's and extra 2134's too! (2x each) Also some other opamps I can't remember the # of right now that I was probably going to play with rolling.

Thank you again for taking the time out of your busy schedule to post pics, Mark! Mucho appreciated.

-Phil

I could swing both ways for the 2308 or 2134, both are good. If you build a CMoy with the 2134 and use a differential PS (easy to do with two resistors and two caps) then you do t need output caps. That is an advantage though possibly inaudible.

Both chips drive my 700S with ease and really do sound very good. My 700s are 32ohms. I think going to 18V on the chip gives more drive and headroom and is easy enough to do.

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Projects:“MWH” - 1920s UX-171A preamp | ”Image” - Low Profile 100W Class D amp | ”fiih” - 150W Holton inpsired power amp | "Precision" - 300W Class AB Holton power amp


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PostPosted: 19 Mar 2020, 22:47 
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e_phil_b wrote:
Also some other opamps I can't remember the # of right now

ON Semicoductor LM833NG's. Actually found the old forum post where someone directly replaced the 2134 for this with supposed pleasing results. (and my reasoning for purchase) A different application altogether but perhaps a drop in replacement for rolling? (close specs?) In other words, all the smoke should stay in?

https://www.onsemi.com/pub/Collateral/LM833-D.PDF


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