Biasrocks
Well-known member
rickc said:
I believe Abbey Road gear was designed to work into a 200 Ohm load.
Regards,
Mark
EMI RS124
- Thread starter ed rees
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Help Support GroupDIY Audio Forum:
I believe Abbey Road gear was designed to work into a 200 Ohm load.
Regards,
Mark
with a 200R load, the output steps are:
0 -5 -10 -15 -20 -30
-5.3 -12 -20.7 -25.7 -36.2 0ff
and the input impedance varies from 368R to 168R, which is better
and if we take the -5.3dB away as a termination loss, then things are slightly better.
I view what was done as crude, and have trouble believing that it originated at Abbey Road.
0 -5 -10 -15 -20 -30
-5.3 -12 -20.7 -25.7 -36.2 0ff
and the input impedance varies from 368R to 168R, which is better
and if we take the -5.3dB away as a termination loss, then things are slightly better.
I view what was done as crude, and have trouble believing that it originated at Abbey Road.
from a purely electronic standpoint, yes. But the unknown is how much signal is required at the 6BC8 grids for some amount of GR.
The peerless original was likely a stepup, so there was "free" gain there.
Someone should measure theirs to see what the stepup is, and then back the turns ratio out of it.
I suspect that in a +4 system, the 1540 will work. It will also give you a bridging input, rather than a 600R one.
The peerless original was likely a stepup, so there was "free" gain there.
Someone should measure theirs to see what the stepup is, and then back the turns ratio out of it.
I suspect that in a +4 system, the 1540 will work. It will also give you a bridging input, rather than a 600R one.
I used a 1:15 on mine and have to use a ~ 20db pad for line sources. As long as the source has some attenuation capability this gives a nice range to adjust with the input trim pot. Based on that, it seems like a 1:1 would fare well unless you got into a situation where your source was lacking enough gain. For the typical output level of a preamp it would seem sufficient and you'd probably even want a little control there even with a 1:1.
dale116dot7
Well-known member
I haven't been on here for like a couple years. I got totally burnt out doing mic capsules and sorta disappeared for a while. When I first looked at the 'wrong' schematic, I sat there tracing out the current flows and came up with around a 45k plate load on the GR tube and around 105k for the feedback resistors for the DC voltages to all work. So I guessed 47k and either 100k or 120k. It was actually pretty easy. Haven't built one, I used an interstage transformer like the UA175 for my 'altec-like' build. This might be an interesting project.....
Biasrocks
Well-known member
Hey Dale,
Great to have you back!
Regards,
Mark
Great to have you back!
Regards,
Mark
jensenmann
Well-known member
IIRC the above posted output pad is the circuit used in the "hidden" VK1 clone project, which is more or less an Altec 436 on steroids. There were two versions of the output pad, one working and one not so good working (wrong steps, wrong load). Now don´t ask me if this is the good one. Just wanted to add "be careful".
thinking about the bridged tee attenuator that I designed (see earlier post), using break before make switches could be bad, as it occurs to me that when the series arm opens, that's the condition for infinite loss... but it's only infinite loss if the shunt arm gets to ground. Since (when the switch is operated), the series arm goes to open, and the shunt arm goes to open, then there's two resistors equal to the design impedance connected between input and output.
that could be loud, as that is not quite zero attenuation. Build one and see. You don't need to go nuts and buy 1% resistors, use nearest 5%. The attenuation values won't be quite right, and the impedance of the pad won't be constant, but it'll be close enough to confirm my suspicions.
that could be loud, as that is not quite zero attenuation. Build one and see. You don't need to go nuts and buy 1% resistors, use nearest 5%. The attenuation values won't be quite right, and the impedance of the pad won't be constant, but it'll be close enough to confirm my suspicions.
I'd like to use the original attenuator (as in the original EMI schematic posted by Winston O' Boogie) but I can't understand how to connect it (in and out) ... this is my limit because I have not too experience with these circuits
FabLab,
I'm looking at the schematic posted by o'boogie, that says 'rotheu' in the title block.
I don't know what you're lacking, the output terms are clearly marked.
maybe you don't know how to connect the 436 to the attenuator?
the schematic says to connect the output transformer for 150R output.
Refer to the 436 schematic, as published by Altec
The transformer is drawn the same way as it is in the 'rotheu' schematic
for 150R, strap 1 and 2, that goes to the top 160R resistor in the 'rotheu' schematic
strap 3 and 4, that goes to the bottom 8R resistor in the 'rotheu' schematic
If you can't find the output terminals, then I can't help you further.
I'm looking at the schematic posted by o'boogie, that says 'rotheu' in the title block.
I don't know what you're lacking, the output terms are clearly marked.
maybe you don't know how to connect the 436 to the attenuator?
the schematic says to connect the output transformer for 150R output.
Refer to the 436 schematic, as published by Altec
The transformer is drawn the same way as it is in the 'rotheu' schematic
for 150R, strap 1 and 2, that goes to the top 160R resistor in the 'rotheu' schematic
strap 3 and 4, that goes to the bottom 8R resistor in the 'rotheu' schematic
If you can't find the output terminals, then I can't help you further.
Yes Rickc, I have some problems to connect the terminals in the attenuator
it is not clear (for me) where is the output in the schematic
The input seems to be connected to the node of 156 ohm and 6.5 ohm, but where is the output?
Do you think is necessary the 160 ohm resistor? I have read that this resistor is there to keep the impedance in 200 Ohm
it is not clear (for me) where is the output in the schematic
The input seems to be connected to the node of 156 ohm and 6.5 ohm, but where is the output?
Do you think is necessary the 160 ohm resistor? I have read that this resistor is there to keep the impedance in 200 Ohm
Attachments
ok. pictures make things mo betta!
ok. the 160R resistor "builds-out" the output impedance of the unit to 200R. That means that the 436, with the output strapped for 150R has an actual source impedance of 40R, so we add 160R to it and voila! the output source impedance is now 200R. That's necessary so that the steps on the attenuator follow the markings.
The output is at the 6R5 resistor (lo) and the junction of the 4323R and 200R resistors (hi)
ok. make sense now? Note my previous post about make before break switches. It does matter (see my post). The critical one (important) is the one that ends in 6R5.
Please tell me where to find the rest of this schematic.
ok. the 160R resistor "builds-out" the output impedance of the unit to 200R. That means that the 436, with the output strapped for 150R has an actual source impedance of 40R, so we add 160R to it and voila! the output source impedance is now 200R. That's necessary so that the steps on the attenuator follow the markings.
The output is at the 6R5 resistor (lo) and the junction of the 4323R and 200R resistors (hi)
ok. make sense now? Note my previous post about make before break switches. It does matter (see my post). The critical one (important) is the one that ends in 6R5.
Please tell me where to find the rest of this schematic.
Fablab,
ok. I had the schematic, but really didn't know what drawing you were talking about initially. There are several already posted in this thread...
It always helps to post a link to the drawing that you're looking at so others don't have to chase it down, etc.
I hope you're on the path to success now.
ok. I had the schematic, but really didn't know what drawing you were talking about initially. There are several already posted in this thread...
It always helps to post a link to the drawing that you're looking at so others don't have to chase it down, etc.
I hope you're on the path to success now.
