Passive Preamp Mod help

mantha3 · 29th September 2011, 01:13 PM

Hi,

I'm thinking about making a mod to my passive preamp... I’m not an engineer and wanted to bounce my idea off of the group. My passive drives a tube amp and the problem I’m having is something I think tube amp people may have seen and can relate to so I put this question on this forum. I appreciate any time in talking on this topic all.

Background.
I have a home built passive preamp. This passive pre has a 100 K attenuator and it is routed to a pair of outputs. I built the passive so I could connect my tube amp and a sub woofer powered by a plate amp. If I have just the tube amp connected the sound is great and the unit works well. If I am running the system with just the tube amp and then connect to the passive on the 2nd set of the sub woofer the subwoofer sort of sucks the life out of the tube amp. Things sound fine but the volumn level on the tube amps drops a decent amount. I have a second tube amp that I’ve tested with.. If I connect the 2nd tube amp the issue I described with the plate amp of the sub is not happening

Issue –
I think the impedance of the tube amp and the plate amp on the subwoofer is the issue. I think the volts of the signal from the passive pre get sucked into the subwoofer where the volts can flow less restricted.

Possible cheap fix –
I found some talk on this forum that I think is related (I think)… I read some items that make me think 22k or 47k resistors on the (+) wire to the ouput RCA jacks may be the fix. I think I’d just need four resistors… Disconnect the (+) wire connected to one of the ouput RCA jacks and insert between the (+) wire and the (+) solder tab of the ouput RCA the resistor. I’d do this for the R and L of each channel. The idea here is that the resistors will isolate the two output channels from one another and the impedance bug won’t be so apparent.

I appreciate the help of the DIY group here… Again, I put this on the Tube Amp forum as I thought the tube amp folks may have experience with this given the impedance of tube amps…

sakis · 29th September 2011, 01:23 PM

i think that it might help but will not solve the problem totally

the best way is to probably drive a Buffer in a way to isolate the problem of impedance...

since its for a sub almost anything will do the job .... stay with this .... a passive 100k to a source that has very high impedance is the cleanest you will ever get dont destroy it with unrelated signals or impedance

( thanks to your amplifier's impedance except clean you loose very litle of the dynamics in low power ...this is importand to keep and this is why solid state people with amplifier impedance of 22-47K cannot be happy with passives or even B1 buffers ...in low listening levels more kick is required ... )

mantha3 · 29th September 2011, 04:15 PM

Ok, I think I got ya. run a buffer stage in the circuit.

So in my circuit... Do just run a buffer between the source and the passive preamp? or between the passive and the tube amp?

Yes, newbie here

sakis · 29th September 2011, 04:30 PM

keep the passive as is .... use a parallel out from the passive to drive a buffer and then from the out of the buffer drive your sub

SY · 29th September 2011, 04:51 PM

Or, if you're ambitious, use a high quality buffer to drive both- 100k is pretty high for a passive, and any cable capacitance can cause HF errors. I've published a good tube buffer, and there are no doubt many other good designs out there.

mantha3 · 13th October 2011, 08:54 PM

Sakis, SY,

I've been doing some research on buffer stages... I agree a buffer from the passive to the Sub Woofer would be a help. Since this buffer would not be in the signal to my Tube Amp I'm thinking of a real basic PASS B1 Buffer Between the Passive and the Sub. Cheaper and I don't think I'd really hear the benefits of a tube buffer out of the sub.

???

I appreciate the advice!

SY · 13th October 2011, 09:49 PM

That would work fine. So would a cheap opamp buffer. Whatever is easiest for you.

Pano · 13th October 2011, 09:53 PM

Quote:

Originally Posted by SY

I've published a good tube buffer...

Were? Is it on public display on the interwebs?

SY · 13th October 2011, 10:11 PM

That's "teh interwebs," dude.

pinkmouse · 13th October 2011, 11:10 PM

T' interweb, happen...

Don't bother Michael, it doesn't sound of anything, just sits there and glows.

29th September 2011, 01:13 PM	#1
mantha3 diyAudio Member Join Date: Oct 2010	Passive Preamp Mod help Hi, I'm thinking about making a mod to my passive preamp... I’m not an engineer and wanted to bounce my idea off of the group. My passive drives a tube amp and the problem I’m having is something I think tube amp people may have seen and can relate to so I put this question on this forum. I appreciate any time in talking on this topic all. Background. I have a home built passive preamp. This passive pre has a 100 K attenuator and it is routed to a pair of outputs. I built the passive so I could connect my tube amp and a sub woofer powered by a plate amp. If I have just the tube amp connected the sound is great and the unit works well. If I am running the system with just the tube amp and then connect to the passive on the 2nd set of the sub woofer the subwoofer sort of sucks the life out of the tube amp. Things sound fine but the volumn level on the tube amps drops a decent amount. I have a second tube amp that I’ve tested with.. If I connect the 2nd tube amp the issue I described with the plate amp of the sub is not happening Issue – I think the impedance of the tube amp and the plate amp on the subwoofer is the issue. I think the volts of the signal from the passive pre get sucked into the subwoofer where the volts can flow less restricted. Possible cheap fix – I found some talk on this forum that I think is related (I think)… I read some items that make me think 22k or 47k resistors on the (+) wire to the ouput RCA jacks may be the fix. I think I’d just need four resistors… Disconnect the (+) wire connected to one of the ouput RCA jacks and insert between the (+) wire and the (+) solder tab of the ouput RCA the resistor. I’d do this for the R and L of each channel. The idea here is that the resistors will isolate the two output channels from one another and the impedance bug won’t be so apparent. I appreciate the help of the DIY group here… Again, I put this on the Tube Amp forum as I thought the tube amp folks may have experience with this given the impedance of tube amps…

29th September 2011, 01:23 PM	#2
sakis diyAudio Member Join Date: Jan 2006 Location: nea makri athens greece	i think that it might help but will not solve the problem totally the best way is to probably drive a Buffer in a way to isolate the problem of impedance... since its for a sub almost anything will do the job .... stay with this .... a passive 100k to a source that has very high impedance is the cleanest you will ever get dont destroy it with unrelated signals or impedance ( thanks to your amplifier's impedance except clean you loose very litle of the dynamics in low power ...this is importand to keep and this is why solid state people with amplifier impedance of 22-47K cannot be happy with passives or even B1 buffers ...in low listening levels more kick is required ... ) __________________ SERVICE ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr

29th September 2011, 04:30 PM	#4
sakis diyAudio Member Join Date: Jan 2006 Location: nea makri athens greece	keep the passive as is .... use a parallel out from the passive to drive a buffer and then from the out of the buffer drive your sub __________________ SERVICE ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr

29th September 2011, 04:51 PM	#5
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	Or, if you're ambitious, use a high quality buffer to drive both- 100k is pretty high for a passive, and any cable capacitance can cause HF errors. I've published a good tube buffer, and there are no doubt many other good designs out there. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

13th October 2011, 09:49 PM	#7
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	That would work fine. So would a cheap opamp buffer. Whatever is easiest for you. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

29th September 2011, 04:15 PM	#3
mantha3 diyAudio Member Join Date: Oct 2010	Ok, I think I got ya. run a buffer stage in the circuit. So in my circuit... Do just run a buffer between the source and the passive preamp? or between the passive and the tube amp? Yes, newbie here

13th October 2011, 08:54 PM	#6
mantha3 diyAudio Member Join Date: Oct 2010	Sakis, SY, I've been doing some research on buffer stages... I agree a buffer from the passive to the Sub Woofer would be a help. Since this buffer would not be in the signal to my Tube Amp I'm thinking of a real basic PASS B1 Buffer Between the Passive and the Sub. Cheaper and I don't think I'd really hear the benefits of a tube buffer out of the sub. ??? I appreciate the advice!

13th October 2011, 10:11 PM	#9
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	That's "teh interwebs," dude. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

13th October 2011, 11:10 PM	#10
pinkmouse diyAudio Moderator Join Date: Apr 2002 Location: Chatham, England	T' interweb, happen... Don't bother Michael, it doesn't sound of anything, just sits there and glows. __________________ Al I conceive of nothing, in religion, science or philosophy, that is more than the proper thing to wear, for a while. Charles Fort

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