Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: embotone on September 16, 2013, 04:18:28 pm
-
I am building an single channel Ampeg B-25 clone (guess that doesn't make it a clone), and want to have a balanced pre amp out to run to a board if necessary. The schematic shows an unbalanced out, but I would like to have a balanced out. I am not sure what transformer to use for this, does anyone have any specific suggestions? Weber has one, but it doesn't have great low end frequency response: https://taweber.powweb.com/store/balun.jpg
-
I am building an single channel Ampeg B-25 clone (guess that doesn't make it a clone), and want to have a balanced pre amp out to run to a board if necessary. The schematic shows an unbalanced out, but I would like to have a balanced out. I am not sure what transformer to use for this, does anyone have any specific suggestions? Weber has one, but it doesn't have great low end frequency response: https://taweber.powweb.com/store/balun.jpg (https://taweber.powweb.com/store/balun.jpg)
lots of threads about balanced outputs vs. unbalanced etc, lately.
To avoid impedance issues of grafting in another load at the Ext. Amp. Jack on that B25 schematic, I'll second Weber's suggestion of using the speaker output to get the signal. you can put a 2Kohm load , or even a 200ohm load in parallel with an 8ohm speaker and not have to worry about impedance impact of the speaker load.
see this:
http://www.jensen-transformers.com/as/as045.pdf (http://www.jensen-transformers.com/as/as045.pdf)
as far as low freq. response, except to pay$$. Try Cinemag, Jensen, Sowter, or Lundahl.
If sticker shock deters,, consider going back to unbalanced:
(http://i.imgur.com/taoSltS.png)
let R2 be the Zout you want, and figure the voltage divider ratio (and thus R1) for a given +dBu level you want to send.
If you stick to your balanced output guns, include a ground lift on the output (XLR pin#1).
-
> schematic shows an unbalanced out
It is over 100K impedance. You can't get good response from a transformer with 100K impedance (or not without spending too much).
Use a buffer. Or tap the speaker output which can drive most anything.
-
The following circuit will work if driven from a low impedance source like an op amp; or you might be able to drive it directly from the output transformer secondary.
(http://)
-
Most whirlwind direct boxes have a Jensen transformer. Cheap in Ebay, just make sure the model has the correct one. Check whirlwind specs per model. It will have mostly all the parts you will need. I bought one for $22 shipped.
-
A tap off of the speaker output through an inexpensive DI box like Ed mentioned sure seems like an easy way to go. I have room in my chassis to install the whirlwind components internally but don't think it is worth the effort. Thanks everyone.
-
This may be worth considering. It's not a transformer solution, but very simple. Requires dual polarity 16v power supply. Look at the Balanced output of this Ampeg BA115.
See page 2 of the schematic in the first post of this thread.
http://www.el34world.com/Forum/index.php?topic=16076.0 (http://www.el34world.com/Forum/index.php?topic=16076.0)
-
ola dudes; I like the Sluckey idea mucho. Uses a nice tidy TL072 op amp chip that costs 80 cents! bingo! It's two, two, two chips in one!
:icon_biggrin:
-
This may be worth considering. It's not a transformer solution, but very simple. Requires dual polarity 16v power supply. Look at the Balanced output of this Ampeg BA115.
See page 2 of the schematic in the first post of this thread.
http://www.el34world.com/Forum/index.php?topic=16076.0 (http://www.el34world.com/Forum/index.php?topic=16076.0)
I assume you are speaking of a separate power supply. Viewing the schematic, it looks as if the circuit is simply connected to existing source. would you reference a sample of a power supply so I can understand how to implement. I am not familiar with a dual polarity 16v PT or how it would be used.
-
Here's the entire circuit. I just cut it from the original so you could see everything together. +12VDC would work fine also.
-
For just one (even a few) chip, here's what always worked for me. Smallest 24VCT transformer you can find, couple caps, gives +/-18V. Pair of 220r-1K resistors and another pair of caps damps the buzz and drops the volts. Teeny caps "at" chips not essential with mild opamps like '072 if the supply (and its caps) is within a foot of the chip.