Improve this Boonton 1120 Output Stage

[jackinnj]

This is a portion of the output stage of the Boonton 1120 THD Analyzer -- uses a pair of NE5534's to obtain single-ended or balanced output -- was thinking to use the AD825/AD826 compound opamp for this purpose -- i have a spare output stage card from a broken 1120 to play with:

[I_Forgot]

It looks like the - inputs of the two final opamps are driven. How do you get balanced output from that?

I_F

[lineup]

jackinnj

I do think, you can have a hard time
by using Op-Amps to try to get the excellent precision
you can get by using a dedicated IC for this.

Analog Devices SSM2142 is such a high performance circuit!
SSM2142 Balance Line Driver Datasheet

Product Description

The SSM2142 is an integrated differential-output buffer amplifier that converts a single-ended input signal
to a balanced output signal pair with high output drive.

By utilizing low noise thermally matched thin film resistors and high slew rate amplifiers, the SSM2142 helps maintain the sonic quality of audio systems by eliminating power line hum, RF interference, voltage drops, and other externally generated noise commonly encountered with long audio cable runs.

Excellent rejection of common-mode noise and offset errors is achieved by laser trimming of the onboard resistors, assuring high gain accuracy.

The carefully designed output stage of the SSM2142 is capable of driving difficult loads,
yielding low distortion performance despite extremely long cables or loads as low as 600 Ohm,
and is stable over a wide range of operating conditions.

Regards
lineup

[jackinnj]

Originally posted by I_Forgot
It looks like the - inputs of the two final opamps are driven. How do you get balanced output from that?

I_F

It's lifted right from the schematic in their service manual -- there is one source input to a series of FET switches.

there's just a wee bit of 2nd order distortion and I am wondering if the NE5534's are up to the task -- it would seem that a mismatch in the opamps could be part of the problem whether or not one chooses the balanced or grounded option.

I have used the SSM2142 -- it's THD+N% is very low (there is a graph in the PDF) -- but it's voltage swing is less than that of the NE5534 combo. I will try it out this afternoon if I get a few minutes.

by the way, the Super-Oscillator from Linear is balanced -- basically two very high speed, tuned (current feedback) opamps run amock.

[jackinnj]

turns out that the schematic I showed -- which is what i copied out of the Boonton Manual -- is for an earlier version. when I popped the top and pulled the output card i saw that the driver was a pair of NE5534's, one a follower, which lead to a relay board.

i have also found out that it makes a difference when you are using the thing -- early in the morning before all the activity in the neighborhood gets going the Boonton's distortion is 0.0008% to 0.0010% -- this with no fluorescents going, no other test equipment running except a Tektronix AA501. in the afternoon or early evening it will average at 0.001% to 0.0014%

i'll leave well enough alone for now. i do appreciate the suggestion of the SSM2142 line driver --

[maxlorenz]

Hi Jackinnj, Lineup,

I am looking for balanced output conversion for my single ended active crossover to combat a little buzz problem.

I thought about opamp based circuit until I saw Lineup's recommendation, wich is best suited for my limited skills.

I've done a little search and found this one, the DRV134, from Burr Brown:
http://docs-europe.electrocomponents...6b80030148.pdf

It is a little cheaper than SSM2142 (at least at RS components; stock nº 378-2676 )
Says "specified for high performance audio"

Regards,
M

[PBB]

Please forgive this request if it is inappropriate. I would like to get a copy of the Boonton 1120 documentation (manual, ops, calibration). Please let me know if this is possible.
Thanks,
PBB

[jackinnj]

there are 3 Boonton 1120's for sale on Ebay at the moment.

if you have problems with the 1120 the first thing to check is the power supply -- according to a tech I spoke with (and there are very few of them around from the time the instrument was manufactured) -- the power supply caps are prone to failure.

[1audio]

I have examples of both the realier and the later version of the 1120. The later version has a completely floating oscillator with opto-isolated connections to the controller. It also has bipolar output transistors to boost the available drive to 50 Ohms source at 16V RMS. It is still at the same .0007-.0009% THD at best. I think the limit is in the oscillator circuit itself.

[1audio]

Jack:
Reading your note more carefully left me wondering if you have the output amp on the back of the instrument?