The reason I want to know about output impedance is that I am trying to design the Joushua Tree resistor set in such a way that it matches well with the XBOSOZ, so I want the input impedance of the attenuator to be a bit higher than the output impedance of the XBOSOZ.
Also It may be an advantage to have the output impedance of the attenuator match the output impedance of the preamp.
In my prototype I used 2.2K output impedance on the attenuator, but I am leaning toward 750 ohms now (the lowest input impedance would be around 1.8K).
Any thoughts?
Also It may be an advantage to have the output impedance of the attenuator match the output impedance of the preamp.
In my prototype I used 2.2K output impedance on the attenuator, but I am leaning toward 750 ohms now (the lowest input impedance would be around 1.8K).
Any thoughts?
Sanity check pays off. 🙂
LTspice rocks. 🙂
Ok so here is the situation with the circuit's DC bias with the BOM Brians and I are using.
V+ = 75V
V- = 20V
R5 = 10K
R6 = 27R
R9 = 221R
D1 = 6.8V zener
IR6 = -90ma
which is 45ma of bias per side, and results in(measured) 28.8V drop accross R1/R6(and R2/R7).
Now my mind is at ease. 🙂
The power resistors are quite warm, but well within sane levels.
LTspice rocks. 🙂
Ok so here is the situation with the circuit's DC bias with the BOM Brians and I are using.
V+ = 75V
V- = 20V
R5 = 10K
R6 = 27R
R9 = 221R
D1 = 6.8V zener
IR6 = -90ma
which is 45ma of bias per side, and results in(measured) 28.8V drop accross R1/R6(and R2/R7).
Now my mind is at ease. 🙂
The power resistors are quite warm, but well within sane levels.
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Member
Joined 2002
jleaman said:
Yes, but just attaching them to the metal case would probably be sufficient. MY HS just get luke warm, just above case temp.
Member
Joined 2002
Russ White said:
I was actually going touse a 1/4" copper bar that is bolted to the case. I think ill test mine when im at brian GT's house and see to determin if i need heat sink's or not. If i remember brian said he got heat sink's from you for this ? With transformers too.!
IIRC, the output impedance of the circuit is much lower than 750 ohms, more in the range of 100 ohms or so.
You are correct that you want the input impedance of the Joshua Tree to be higher than the output impedance of the circuit, and you also want the output impedance of the Joshua Tree to be a lower than the input impedance of the amplifier being drives.
When Nelson designed the original BOSOZ, he decided a 5K potentiometer was a good balance. In my preamp, I used a 24 position series attenuator with a 10k impedance and it works very nicely with the Susy GC design, and my Aleph-X's.
On your Joshua Tree I think you are moving in the right direction. The Twisted BoSoZ will laugh at driving a 1.8k input impedance (it will grin happily and tell you to bring it on!), and a 750 ohm output impedance will be great to drive pretty much any amp you come across, including the GC Susy.
I coupled my FET's to the case bottom panel, which is less than 1/16th inch thick aluminum, and it is more than enough heatsinking for them. The bottom panel gets warm, but only slightly so.
Cheers, Terry
Hi Guys,
Did the original BoSoZ use a stepped attenuator of just a 5K pot?
Qhat are the pros and cons of using a pot vs an attenuator?
Thanks, Terry
Did the original BoSoZ use a stepped attenuator of just a 5K pot?
Qhat are the pros and cons of using a pot vs an attenuator?
Thanks, Terry
still4given said:
To use a pot in balanced mode you would need a 4 deck pot, or you would have to somehow mechanically couple (gears or pullys or something) 2 x 2 deck pots or 4 x 1 deck pots. 4 deck pots are expensive, and tracking between decks can sometimes be downright aweful.
That is why a stepped attenuator is a great choice, since linear tracking between channels is nearly perfect as long as you use reasonably matched(say 1%) resistors on all four signal channels.
For single ended use a dual gang pot is not a bad choice, but I actually like an attenuator much better, partly because the resistor element of a pot is a weak link, and it subject to severe degradation with wear. An attenuators resistors can be much higher quality(metal film as opposed to the carbon element of a pot) and are not subjected to the same kind of abuse.
Hope that helps.
Cheers!
Russ
Hi Russ,
Thanks for the explanation. So a pot works but may or may not give the same quality of sound?
I thought I might like to test the XBOSOZ before I spend another $100 on it. I believe I have some 5K or 10 pots on hand. I could hook it up SE to get an idea of how it sounds. A friend said I could use 10K if I reduce R20 and R21. True?
As far as Dual pots, can you use a stereo pot for each side and no balance control? I'm thinking of using a pair of controls from an old TASCAM tape deck that has a belt drive between the two. Might work?
Blessings, Terry
Thanks for the explanation. So a pot works but may or may not give the same quality of sound?
I thought I might like to test the XBOSOZ before I spend another $100 on it. I believe I have some 5K or 10 pots on hand. I could hook it up SE to get an idea of how it sounds. A friend said I could use 10K if I reduce R20 and R21. True?
As far as Dual pots, can you use a stereo pot for each side and no balance control? I'm thinking of using a pair of controls from an old TASCAM tape deck that has a belt drive between the two. Might work?
Blessings, Terry
still4given said:
If you use the pot on output and drive a load (power amp) with an input impedance of greater than 10K you can use your 10K pot without issue. In fact that is what I did when I first test mine.
A word of advice. Single ended the XBOSOZ does not sound great with the POT on input. Use the pot on output.
Some of you may be intersted in this:
http://www.diyaudio.com/forums/showthread.php?postid=868700#post868700
I need something DIY and balanced to feed my XBOSOZ. 🙂
http://www.diyaudio.com/forums/showthread.php?postid=868700#post868700
I need something DIY and balanced to feed my XBOSOZ. 🙂
Great stuff
Guys,
Recieved my kit today (parts, boards, and x-formers.) Incredible job on everything.
Thanks!
ryan
Guys,
Recieved my kit today (parts, boards, and x-formers.) Incredible job on everything.
Thanks!
ryan
Member
Joined 2002
I'm at briangt's house i arried lastnight ill be taking pics of my kit and parts.
Brian/Russ REALLY nice job on the kit packaging andother goodies. I'll take pictures of each step if you like.
Jase
Brian/Russ REALLY nice job on the kit packaging andother goodies. I'll take pictures of each step if you like.
Jase
Case Wiring
Here is how I have my case wired for now until the Joshua Tree PCBs get here. Until then I have my external attenuator (same design just bigger).
Yes, There is a fuse(1A slow blow) inline with the power cord. 🙂 No, I am not that nuts. 😀
Just thought it might help people with ideas.
Cheers!
Russ
Here is how I have my case wired for now until the Joshua Tree PCBs get here. Until then I have my external attenuator (same design just bigger).
Yes, There is a fuse(1A slow blow) inline with the power cord. 🙂 No, I am not that nuts. 😀
Just thought it might help people with ideas.
Cheers!
Russ
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Russ... Vaughan Williams is the nuts! Like the fact that the sinks are inward in the case... I will do that with mine.
That's a beautiful case and layout Russ! I'm hoping to order my XBOSOZ and attenuator soon (just waiting to finish up school over the next month). I can't wait!
-Scott
-Scott
OT again. My above OT comment I placed in this thread needs correction. I cut R13 (bias res.) out on the A30 ...BAM!.... BAM there it was! As good as the mini...just louder.
OK.. carry on.......🙂
Wonder where those boards are! 😉 I may get my X-GC done before I have a pre to drive it! Ahh, what shall I do?! 🙂
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