Last evening I finished the PCB for the replacement amplifier board for my HP465A test amplifier. The 465A had gone wonky, with a lot of noise and a DC offset problem.
The circuit which is attached uses the SSM2019 programmable gain amplifier for 20, 40 and 60dB gain (trimmable), and follows an idea which Jung published in AudioXpress in 1995 and similar designs found on the Analog Devices website. It has "belt and suspenders" input protection -- perhaps unnecessary since the SSM2019 seems to have this incorporated.
The chip Walt used is no longer in production (or at least I couldn't find it) and almost all devices are SMT. If there is interest I could send the boards out for production and distribute at cost. The proto-type has the option for using the Linear Tech LT1964 and LT1761 ultra-quiet regulators (shown on the schematic) AND a TL431 type regulator. The proto's fit exactly into the space allotted in the Hewlett Packard 465A box. The power coming off the HP transformer is high so this had to be tamed with an active zener. If anyone is interested in testing power supplies, this is a handy instrument to have, and the HP 465A power supply, chasis etc. makes it convenient for DIYrs.
The circuit which is attached uses the SSM2019 programmable gain amplifier for 20, 40 and 60dB gain (trimmable), and follows an idea which Jung published in AudioXpress in 1995 and similar designs found on the Analog Devices website. It has "belt and suspenders" input protection -- perhaps unnecessary since the SSM2019 seems to have this incorporated.
The chip Walt used is no longer in production (or at least I couldn't find it) and almost all devices are SMT. If there is interest I could send the boards out for production and distribute at cost. The proto-type has the option for using the Linear Tech LT1964 and LT1761 ultra-quiet regulators (shown on the schematic) AND a TL431 type regulator. The proto's fit exactly into the space allotted in the Hewlett Packard 465A box. The power coming off the HP transformer is high so this had to be tamed with an active zener. If anyone is interested in testing power supplies, this is a handy instrument to have, and the HP 465A power supply, chasis etc. makes it convenient for DIYrs.
Attachments
I use the "W" version of the SSM2019 -- easier for me to hand-solder. I was just thinking of just having the boards made -- I used the SSM2019 since I had them on hand. Using this chip does "bandwidth limit" the project a bit (compared to the HP465A which is flat to 1MHz).
I was wondering whether to do some active filtering on the output. so that the noise can be referenced to a specific bandwidth.
Right now I am having an arguement with my PCB software -- it isn't allowing me to place the PowerPlane where I want it so I have had to "pour copper".
I was wondering whether to do some active filtering on the output. so that the noise can be referenced to a specific bandwidth.
Right now I am having an arguement with my PCB software -- it isn't allowing me to place the PowerPlane where I want it so I have had to "pour copper".
I added a 12.5 Hz Chebyshev low-pass filter to the circuit -- you can jumper it in or out with a couple of molex connectors (i.e. if you want to measure power supply noise, the filter is wired in, if you want to measure to 1 MHz, the filter is out.) I also changed the input caps to "back to back" 1000 uF electrolytics -- thus the response should be 0.1 Hz to 10Hz. This necessitated going from surface mount to through-hole.
I also removed the protection diodes -- and will rely on the diodes on the SSM2019.
Jack
I also removed the protection diodes -- and will rely on the diodes on the SSM2019.
Jack
here's a link to the article --
http://www.tech-diy.com/hp465a_amplifier_modernization.htm - for purposes of clarity the ground plane isn't shown in the PCB design.
the jury is out on the LTC1761 and LTC1964 ultra-low noise regulators -- they may be unnecessary (and as I state, just add to the cost and complexity.)
C1, C4 and C7 have been omitted. if there is an RF or EMI problem they can be soldered in.
i will add a clipping indicator, am thinking of changing the resistor arrangement so that a trimpot can be inserted for the 20, 40 and 60dB ranges -- for the moment I have been soldering in different SMT resistors.
the SSM2019 does run "hot" -- the initial tests show that it is extremely quiet. I did find a cookie tin !
http://www.tech-diy.com/hp465a_amplifier_modernization.htm - for purposes of clarity the ground plane isn't shown in the PCB design.
the jury is out on the LTC1761 and LTC1964 ultra-low noise regulators -- they may be unnecessary (and as I state, just add to the cost and complexity.)
C1, C4 and C7 have been omitted. if there is an RF or EMI problem they can be soldered in.
i will add a clipping indicator, am thinking of changing the resistor arrangement so that a trimpot can be inserted for the 20, 40 and 60dB ranges -- for the moment I have been soldering in different SMT resistors.
the SSM2019 does run "hot" -- the initial tests show that it is extremely quiet. I did find a cookie tin !
I updated the PCB -- now have a dual-comparator set up as a clipping indicator (over=>red, under=>green).
I tested the Linear Tech low noise LDO regulators -- they tame what was already a pretty quiet supply with the TL431 pre-regulator , but I don't know that they add to performance in any meaningful way.
Since I have just a little more space on the board I was thinking of gilding the lilly and adding an RMS detector...or should I put in a DPDT relay for DC-coupling (and by shorting the 1000uF input caps.)
http://www.tech-diy.com/hp465a_amplifier_modernization.htm
I tested the Linear Tech low noise LDO regulators -- they tame what was already a pretty quiet supply with the TL431 pre-regulator , but I don't know that they add to performance in any meaningful way.
Since I have just a little more space on the board I was thinking of gilding the lilly and adding an RMS detector...or should I put in a DPDT relay for DC-coupling (and by shorting the 1000uF input caps.)
http://www.tech-diy.com/hp465a_amplifier_modernization.htm
The amp in its current incarnation doesn't seem to add any distortion or noise -- here's a pic -- without the clipping indicators installed yet:
works just fine at 20db and 40db, but the connector for 60db isn't functioning quite right yet:
An externally hosted image should be here but it was not working when we last tested it.
works just fine at 20db and 40db, but the connector for 60db isn't functioning quite right yet:
Hello Jack,
just recently I stumbled over a very similar project as Yours (unless that it works with a ground referenced input only and lacks the differential input) .
It is well documented and I thought maybe You`ll find it interesting (but might well be that You know it already) :
http://tangentsoft.net/elec/meas-preamp/
BTW: as the SSM2019 is an obsolete and hard to find part, it appears to me it should be also possible to substitude it with an INA-103 (Burr-Brown) while getting very similar performance. What do you think?
just recently I stumbled over a very similar project as Yours (unless that it works with a ground referenced input only and lacks the differential input) .
It is well documented and I thought maybe You`ll find it interesting (but might well be that You know it already) :
http://tangentsoft.net/elec/meas-preamp/
BTW: as the SSM2019 is an obsolete and hard to find part, it appears to me it should be also possible to substitude it with an INA-103 (Burr-Brown) while getting very similar performance. What do you think?
cocolino said:
The SSM2019 was chosen as the replacement for the SSM2017 which is obsolete. Last I checked the '2019 was current production. WJ used the '2017 in his Audio Amateur article about 8 years ago. If you wanted greater bandwidth an even better preamp would be a good idea. The '2019 here is used in the wide SMT package.
I used dual BNC jacks on the input -- important to ground one of the inputs if you are going single-ended.
I reworked the preamp design with SSM2019 and choice of two filters. The first filter allows 100mHz to 10 Hz noise measurement, the second (adapted from Linear Tech's apnote 83) has a 10Hz - 100kHz bandwidth. Note the inductors and fusing on the inputs. The SSM2019, at least according to the folks at Analog, has protection clamp diodes on the inputs. I found that they failed. C9 is actually 22uF:
An externally hosted image should be here but it was not working when we last tested it.
I was looking for a noise measurement amp and came across this old thread. While the SSM2019 is OOP, the THAT1510 is a drop in replacement (with reputedly better performance) and can be had in small quantities from http://www.ka-electronics.com/kaelectronics/index.html I'm in no way associated with KA, except as a satisfied customer.
It appears that the pcb layout has disappeared from the tech-diy site. Does anyone have a copy archived?
cheers
Paul
It appears that the pcb layout has disappeared from the tech-diy site. Does anyone have a copy archived?
cheers
Paul
spzzzzkt said:
The SSM2019, according to Analog's site, is still in production. It's the SSM2017 which Walt and Jan used for their 1995 Audio Amateur articles that is out of production.
One of the things which I learned (the hard way) was that you need an automatic mechanism to "precharge/discharge" the blocking capacitors -- this is done manually on the Tektronix 7A22 and 5A22 differential amplifiers. I've used the amplifier to measure the noise on a couple of high voltage regulators (the Maida regulator, Last PAS and LR8N3).
I would also use a front-end for the amplifier consisting of a low noise JFET preamp...and the power supply should be contained in a separate box, or some NiCads used to power the device.
For audio, you might want to take a look at my article in AX "The Noise (Analysis) Machine" -- I describe the use of Linear Tech's True RMS Detectors, and rebuilt the filter used by Dolby for his article.
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