I used a 500k dual gang PEC carbon pot for my Marantz 7C copy build but unfortunately, as I had heard others warn about, the tracking of the two tracks is off quite a bit and requires rebalancing every time I reduce volume any where near 1/4 turn or lower.
I noticed that Parts Express and some other suppliers I've used in the past does not carry anything over 250K in an Alps stereo pots which I wanted to try. Any suggestions for another brand that is good enough for the task in a reasonable price range. I guess the next question for anyone versed in old tube designs such as the Marantz 7C, would a 250K pot work for the task or would it cause any impedance mismatching enough to avoid?
I noticed that Parts Express and some other suppliers I've used in the past does not carry anything over 250K in an Alps stereo pots which I wanted to try. Any suggestions for another brand that is good enough for the task in a reasonable price range. I guess the next question for anyone versed in old tube designs such as the Marantz 7C, would a 250K pot work for the task or would it cause any impedance mismatching enough to avoid?
Log or lin?
If log then see 'fake log':
https://sound-au.com/project01.htm#s1
Read it and make your own mind up.
If log then see 'fake log':
https://sound-au.com/project01.htm#s1
Read it and make your own mind up.
Log. I looked at the Elliot Sound article and looks like a potential option for me.
I happened upon a thread where someone was needing to replace the volume pot on a Marantz 7T and in some of the responses, it was thought that as long as the tape head and microphone inputs were not used, that a 250K pot would be appropriate. Not sure why those input sources would change the input impedance to the pot at that stage in the circuit., Do you think a 250K log Alps might work? Or, do I need to stay with 500K?
I happened upon a thread where someone was needing to replace the volume pot on a Marantz 7T and in some of the responses, it was thought that as long as the tape head and microphone inputs were not used, that a 250K pot would be appropriate. Not sure why those input sources would change the input impedance to the pot at that stage in the circuit., Do you think a 250K log Alps might work? Or, do I need to stay with 500K?
The Marantz 7C has a phono stage output cathode follower, so the circuit is not highly sensitive to loading.
But lower than design loads on the phono stage (mainly the balance and volume controls)
will roll off the bass further, due to the 0.47uF output coupling capacitor.
Whether the source is phono, mic, or tape, in any event the source is not relevant, they are mistaken.
Beware of metric pots that have a different diameter bushing and shaft, or too long shaft length.
But the correct Alps pot will track better than your stock pot. Be sure to check the Alps spec for tracking.
But lower than design loads on the phono stage (mainly the balance and volume controls)
will roll off the bass further, due to the 0.47uF output coupling capacitor.
Whether the source is phono, mic, or tape, in any event the source is not relevant, they are mistaken.
Beware of metric pots that have a different diameter bushing and shaft, or too long shaft length.
But the correct Alps pot will track better than your stock pot. Be sure to check the Alps spec for tracking.
I appreciate the feedback on the issue with bass roll off in phono.
I had already assumed I would need to figure out a thin shim to add shaft diameter for the 6mm shaft to mate up with my 1/4” knob bore.
What’s your thought on me using a 250k fixed resistor in series with the 250 k alps pot? I know it would limit max output but I never move beyond 1/2 rotation anyway.
I had already assumed I would need to figure out a thin shim to add shaft diameter for the 6mm shaft to mate up with my 1/4” knob bore.
What’s your thought on me using a 250k fixed resistor in series with the 250 k alps pot? I know it would limit max output but I never move beyond 1/2 rotation anyway.
It would be ok to just use the 250k control without an extra series resistor. The phono bass roll off will still be subsonic.
If the shaft is too long, don't clamp the pot body when cutting it off. That will damage the mechanism inside the pot,
from the force on the shaft. Instead, clamp the end of the shaft, leaving the pot body free while cutting.
If the pot body is not completely sealed, seal it in a small plastic bag with tape around the bushing,
to keep filings out of the inside of the pot.
If the shaft is too long, don't clamp the pot body when cutting it off. That will damage the mechanism inside the pot,
from the force on the shaft. Instead, clamp the end of the shaft, leaving the pot body free while cutting.
If the pot body is not completely sealed, seal it in a small plastic bag with tape around the bushing,
to keep filings out of the inside of the pot.
Good to hear the 250k will only affect subsonic! I will give it a try. Fortunately, I've got lots of experience modifying potentiometer shafts over the years and yes, only way is to secure the shaft for cutting or grinding and good advice on preventing grinding debris from entering any potential opening.
If you don't have access to a lathe, they are commercially available:
https://www.customworldguitarparts....erts-6mm-metric-size-shaft-to-1-4-6-35mm.html
https://www.customworldguitarparts....erts-6mm-metric-size-shaft-to-1-4-6-35mm.html
I've been without a decent turntable since building the preamp and since replacing the volume control with the Alps 250K but recently was able to give it a listen with a good one. Unfortunately, while there is no lack of bass and seems to have a very flat response in the low end, high frequencies are rolling off some. I've given it lots of listening time with two different carts and both carts show the same rolloff. This is the first time I've been able to give the phono side of my build a real listen. Is there any possibility that going from a 500K pot to the 250K Alps pot could cause this? I'm prepared to rebuild the RIAA circuit to rule out any possibility that I may have caused any damage to the polystyrene caps while soldering them into the circuit. I know that this is probably a long shot since both channels seem to be identical and is not likely that I would have overheated both sets of caps.
So, for an abbreviated question I'll repost without the extra baggage.
Is there any possibility for there to be a noticeable roll off of audible range high frequencies if the RIAA circuit saw a 250K volume pot versus the original 500K load?
Is there any possibility for there to be a noticeable roll off of audible range high frequencies if the RIAA circuit saw a 250K volume pot versus the original 500K load?
If anything, the treble response will only improve with the lower resistance volume control.
Did you measure the RIAA resistors and capacitors, or are they at least 1% tolerance? Is the switch set to RIAA?
Are you using the second "output level" control all the way up? Unless there's too much noise, I would do that.
Did you measure the RIAA resistors and capacitors, or are they at least 1% tolerance? Is the switch set to RIAA?
Are you using the second "output level" control all the way up? Unless there's too much noise, I would do that.
I'll be putting my 7C replica build on the bench soon and will remeasure the resistors but I've been too cheap to purchase a decent LCR meter so can't check the caps. They are polystyrene 1%.
My build's only option is RIAA.
When you state "second output level", are you referring to the two pots in the 7C just prior to the outputs? I did not include those in my build. The only level adjusts are my two single 500K pots (in place of the balance pot) prior to the 250K volume pot. Those are at max output.
My build's only option is RIAA.
When you state "second output level", are you referring to the two pots in the 7C just prior to the outputs? I did not include those in my build. The only level adjusts are my two single 500K pots (in place of the balance pot) prior to the 250K volume pot. Those are at max output.
I sprung for capacitance tester but decided to purchase an inverse RIAA converter to allow me to perform a frequency response test on the preamp prior to disassembling its RIAA caps and resistors. While I'm sure the inverse RIAA converter caps and resistors aren't super precision, its probably close enough to tell me whether there's any major issues with holding a fairly flat response in the upper frequencies. I used my Fluke 8060A true rms meter db function for the db readings. I'm not seeing anything that would indicate a problem so I guess the next step would be to rule out turntable and cart.
Here's the inverse RIAA converter I used: https://www.ebay.com/itm/156559691592?mkevt=1&mkpid=0&emsid=e12052.m43.l1123&mkcid=7&ch=osgood&euid=28fc724d0b03474fb531189f53c49493&bu=45334558371&osub=-1~1&crd=20241227075428&segname=12052
Here's the inverse RIAA converter I used: https://www.ebay.com/itm/156559691592?mkevt=1&mkpid=0&emsid=e12052.m43.l1123&mkcid=7&ch=osgood&euid=28fc724d0b03474fb531189f53c49493&bu=45334558371&osub=-1~1&crd=20241227075428&segname=12052
Just play a line level source through the inverse RIAA into the phono input. Are the highs OK now?
If so, then the cartridge is lacking, or else there is wonky loading on the cartridge.
If so, then the cartridge is lacking, or else there is wonky loading on the cartridge.
I will give that a try. I've got lots of listening time via CD player so will be easy to tell if there's a change in response running it through the RIAA adapter into phono input. Great idea!
After playing several cds through the inverse adapter-phono stage and comparing against playing through line, could not tell any difference. Afterwards, I remembered that I have an old Bob James with Earl Klugh in both CD and vinyl and did some A-B listening through a pair of decent Sony earbuds and only heard a miniscule difference between the two which has me wondering if I've been imagining the lack of upper frequencies. Next step will be doing lots of A-B of the same vinyl and CD through my loudspeakers and see if for some odd reason they show a difference.
After all this testing, I now have an odd ticking on one channel of the preamp at around 6 hz or so, only on phono stage. Sort of sounds like a very faint sound of the main prop on the old vietnam era huey chopper. Has anyone ever heard of anything like this out of a tube preamp? I'll post the question in the proper forum.
After all this testing, I now have an odd ticking on one channel of the preamp at around 6 hz or so, only on phono stage. Sort of sounds like a very faint sound of the main prop on the old vietnam era huey chopper. Has anyone ever heard of anything like this out of a tube preamp? I'll post the question in the proper forum.