I looked at your link, and that PDF is bare of anything that resembles a schematic.
...but, the text does imply that it is DC coupled at the input. Find a decent 2.2µf film cap and put it in there.
...but, the text does imply that it is DC coupled at the input. Find a decent 2.2µf film cap and put it in there.
Hi there Shawn, 3.8 is a reference to the DC 300a service manual. Section 3 page 8.
But as we are all aware the schematics often bear no resemblance to the pcb we have in front of us!
S
But as we are all aware the schematics often bear no resemblance to the pcb we have in front of us!
S
Hi Shawn,
What do you want to measure?
For THD, pick up something like an old HP 331A or better. Measure your oscillator to see how good it is, but you may as well continue using it. You can always built a small wein bridge oscillator to use for THD measurements.
You do need a dummy load resistor of some kind.
-Chris
What do you want to measure?
For THD, pick up something like an old HP 331A or better. Measure your oscillator to see how good it is, but you may as well continue using it. You can always built a small wein bridge oscillator to use for THD measurements.
You do need a dummy load resistor of some kind.
-Chris
I thought T was talkin to me?AndrewT said:
anatech said:
Chris, I want to explore all basic measurments of amplifiers. I was under the impression many measurements could be performed with a basic set up. Can you point to expanded details on the wein bridge and its application? I'll keep digging on google meanwhile. Every time I put a call out for test and measurement around here, not much gives. Is this something nobody does at home with their gear? I thought folks around here would have a lot of information about their own test set ups? I guess it is a secret? I guess I have to do more research on my own, shame on me.
Cheers,
Test set up slacker, Shawn. 🙂
stewartwen said:
Lets say vast differences and huge variances are strongly in effect. The DC300 has been through many changes through out its long production history. I guess that is just part of the fun?
Cheers,
Shawn.
Hi Tom Waits,
3.8 seems to vary with the edition posted in the PDF. I have found the same reference @ 2.5 on page 19/20 on my home PC copy.
The paragraph describes when and how to connect your different inputs and how to avoid the problems inherent in not using filters at the input.
Crown have covered all senarios. Full bandwidth for those that need it and filters for signals otherwise won't work with a DC to near RF amplifier.
3.8 seems to vary with the edition posted in the PDF. I have found the same reference @ 2.5 on page 19/20 on my home PC copy.
The paragraph describes when and how to connect your different inputs and how to avoid the problems inherent in not using filters at the input.
Crown have covered all senarios. Full bandwidth for those that need it and filters for signals otherwise won't work with a DC to near RF amplifier.
Tom Waits, (love your records BTW)
The test gear you have is allready a good start, the Hewlett Packard audio oscillator is a real classic and IS (I think) a wein bridge oscillator (with amplitude stabilisation by means of a light bulb....yes indeedy)
I do not know just how good it's distortion performance is but would expect it to be pretty good. In it's day, 0.1% THD would have been considered amazingly good but I would guess it should do 0.01% or so....maybe another member will be more familiar with the detailed spec for it. It may need some work on it to get it as low as possible. For square wave testing (rise time, checking for ringing= sign of incipient instability amongst other things) you could either get a cheap audio signal generator which although it would not have low enough distortion on the sine wave output would probably be fine for square waves. Or you could build a simple convertor to generate square waves from the output of your HP200 by using a comparator IC or a schmitt trigger circuit (either discrete or using schmitt input logic gates) with a suitable attenuator. Theres plenty of info on the web.
THD meter...buy or build, Bob Cordell has a design on his web site (sorry don't have URL to hand but there are several links to it on this site) or you will find many more on the web. You could search for a site called E-panorama which should have designs for all this and more on it (and lots of pop up ads but it's a useful resource). A certain degree of precision and care in terms of earthing, screening, layout and component choice will be required to get good results so if in doubt......buy?
The rest of your gear should be fine, I like old stuff myself and use several old, valved but well sorted items in my own workshop. It has character and build quality not often seen these days 😉
The test gear you have is allready a good start, the Hewlett Packard audio oscillator is a real classic and IS (I think) a wein bridge oscillator (with amplitude stabilisation by means of a light bulb....yes indeedy)
I do not know just how good it's distortion performance is but would expect it to be pretty good. In it's day, 0.1% THD would have been considered amazingly good but I would guess it should do 0.01% or so....maybe another member will be more familiar with the detailed spec for it. It may need some work on it to get it as low as possible. For square wave testing (rise time, checking for ringing= sign of incipient instability amongst other things) you could either get a cheap audio signal generator which although it would not have low enough distortion on the sine wave output would probably be fine for square waves. Or you could build a simple convertor to generate square waves from the output of your HP200 by using a comparator IC or a schmitt trigger circuit (either discrete or using schmitt input logic gates) with a suitable attenuator. Theres plenty of info on the web.
THD meter...buy or build, Bob Cordell has a design on his web site (sorry don't have URL to hand but there are several links to it on this site) or you will find many more on the web. You could search for a site called E-panorama which should have designs for all this and more on it (and lots of pop up ads but it's a useful resource). A certain degree of precision and care in terms of earthing, screening, layout and component choice will be required to get good results so if in doubt......buy?
The rest of your gear should be fine, I like old stuff myself and use several old, valved but well sorted items in my own workshop. It has character and build quality not often seen these days 😉
Hi jez,
I use a fet stabilized wein bridge oscillator for really low distortion. Right out of Walter Jung's "Op Amp Cookbook". A fixed frequency oscillator has lower distortion than a variable one due to the matching of bridge components.
Shawn,
I do all my work at home. If you want, you are welcome by to see what I can do and what you need to get there. You can make better decisions after that I guess. There are changes I have to make right now as well to extend my abilities.
-Chris
I use a fet stabilized wein bridge oscillator for really low distortion. Right out of Walter Jung's "Op Amp Cookbook". A fixed frequency oscillator has lower distortion than a variable one due to the matching of bridge components.
Shawn,
I do all my work at home. If you want, you are welcome by to see what I can do and what you need to get there. You can make better decisions after that I guess. There are changes I have to make right now as well to extend my abilities.
-Chris
Hi Anatech,
I use a bridged T based oscillator made with an NE5534 which is stabilised by a home brewed LDR/LED opto-Isolator controlled by a precision rectifier and low pass filtering (time constants switch with F range) done with a TL074. It's based on a circuit from an old Audio Amateur issue which is itself based on the Heath IG17.
It works very well 😉 My (again home brew) THD meter can measure down to 0.0014% but the distortion of the oscillator is below the noise floor....when carefully nulled I can unplug the oscillator and the reading doesn't change! Does for me😉
I use a bridged T based oscillator made with an NE5534 which is stabilised by a home brewed LDR/LED opto-Isolator controlled by a precision rectifier and low pass filtering (time constants switch with F range) done with a TL074. It's based on a circuit from an old Audio Amateur issue which is itself based on the Heath IG17.
It works very well 😉 My (again home brew) THD meter can measure down to 0.0014% but the distortion of the oscillator is below the noise floor....when carefully nulled I can unplug the oscillator and the reading doesn't change! Does for me😉
Hi jez,
I know what you mean. One day I'll build a few oscillators set to different frequencies for spot THD testing. These oscillators can perform extrememly well. I haven't tried using an LDR yet. They are supposed to be voltage dependant to some degree.
I use a Leader LDM-171 and I also have an HP 331A THD meter. I'm thinking of going through both meters and redoing the caps. I'm also trying to use a Creative X-Fi Extrememusic card in a computer. I need to do some surgery (4556 output op amp has got to go!)
-Chris
I know what you mean. One day I'll build a few oscillators set to different frequencies for spot THD testing. These oscillators can perform extrememly well. I haven't tried using an LDR yet. They are supposed to be voltage dependant to some degree.
I use a Leader LDM-171 and I also have an HP 331A THD meter. I'm thinking of going through both meters and redoing the caps. I'm also trying to use a Creative X-Fi Extrememusic card in a computer. I need to do some surgery (4556 output op amp has got to go!)
-Chris
4556 
I have never tried the route of using computer sound card and software based set up's, I have my doubts about the levels of spurious rubbish from SMPSU and quantization etc (also the poor op amps and high K ceramic caps etc) but I know people who swear by it. The ease and low price of FFT spectrum analysis by this method is tempting though so I may well give it a try some day soon. It would also be nice to be able to print graphs of frequency response etc, it's got to be easier than old Bruel & Kjaer kit. I guess I'm stuck in the past.... but I think I like it that way!
🙂
My own experiance of LDR stabilisation is that it gives the lowest THD of all commonly used methods, certainly for continuosly variable designs such as the one I am using. I have tried FET stabilised oscillators but even the best of the ones I have breadboarded have not gone much below 0.01%, even with feedback around the FET itself.
If yours is lots lower than this I would be interested as to how you did it.... if it's not a secret 😉
I have never tried the route of using computer sound card and software based set up's, I have my doubts about the levels of spurious rubbish from SMPSU and quantization etc (also the poor op amps and high K ceramic caps etc) but I know people who swear by it. The ease and low price of FFT spectrum analysis by this method is tempting though so I may well give it a try some day soon. It would also be nice to be able to print graphs of frequency response etc, it's got to be easier than old Bruel & Kjaer kit. I guess I'm stuck in the past.... but I think I like it that way!
🙂
My own experiance of LDR stabilisation is that it gives the lowest THD of all commonly used methods, certainly for continuosly variable designs such as the one I am using. I have tried FET stabilised oscillators but even the best of the ones I have breadboarded have not gone much below 0.01%, even with feedback around the FET itself.
If yours is lots lower than this I would be interested as to how you did it.... if it's not a secret 😉
Hi jez,
No secret. Like I said, right out of Walt Jung's op amp cookbook. The secret is to match the parts (R and C) closely. Mine uses 4 TL071CP's. I bet the performance may be better with different op amps. It's using an ECG466 as the control element. I hate ECG parts, I'll install a real part some time. If you buy one of these today, who knows what part you will actually have.
-Chris
Edit: I built this back in 1981. Still works great.
No secret. Like I said, right out of Walt Jung's op amp cookbook. The secret is to match the parts (R and C) closely. Mine uses 4 TL071CP's. I bet the performance may be better with different op amps. It's using an ECG466 as the control element. I hate ECG parts, I'll install a real part some time. If you buy one of these today, who knows what part you will actually have.
-Chris
Edit: I built this back in 1981. Still works great.
OOPS.....Yes you did tell me that 😱
so busy typing my reply I totally forgot. Sorry.
I don't have that book but I do have Walt Jungs "op amp application handbook" (free at Analog Devices seminars 😉 )
I will see if it's in there.
so busy typing my reply I totally forgot. Sorry.
I don't have that book but I do have Walt Jungs "op amp application handbook" (free at Analog Devices seminars 😉 )
I will see if it's in there.
Test gear and such
jez, your comments are great. I love new gear but I have great respect for the old HP units that I have purchased. (the rectification valves[in used condition] are probably worth more than what I paid for each piece of test gear!) I have orriginal service manuals with errata for both units and I have cleaned & calibrated both and replaced a couple valves. Generating square waves to me is just "controlled clipping" and I see no reason why I could not utilize your suggestions. I think you have given me enough scoop to get me in the right direction. May I email you direct with some other questions?
Chris, I will look into the references and resources you and jez have listed and discussed. Your offer is profound and I would seriously like to take you up on it in the next week or so. I could bring the old Sea Hag so you can see it in person. 😀 I will email you.
Cheers,
Shawn.
jez said:
jez, your comments are great. I love new gear but I have great respect for the old HP units that I have purchased. (the rectification valves[in used condition] are probably worth more than what I paid for each piece of test gear!) I have orriginal service manuals with errata for both units and I have cleaned & calibrated both and replaced a couple valves. Generating square waves to me is just "controlled clipping" and I see no reason why I could not utilize your suggestions. I think you have given me enough scoop to get me in the right direction. May I email you direct with some other questions?
anatech said:
Chris, I will look into the references and resources you and jez have listed and discussed. Your offer is profound and I would seriously like to take you up on it in the next week or so. I could bring the old Sea Hag so you can see it in person. 😀 I will email you.
Cheers,
Shawn.
DC offset - discharging cap(s)
When I first hooked up the DC300A for test I did not have the volume pots connected. As it turns out, the DC offset set up and procedures rely on these pots to set it correctly as the volume controls are actually part of the offset circuit. Things seem to be running even better since last tuning it yesterday with the orriginal worn-out pots. I am currently looking for new ones. Can I dismantle a couple of alps stereo pots and set them up as singles? They look modular in construction (stacked)?
I have one last known issue. There is a horrible sound of a cap discharging when I turn the unit off. Any thoughts? What should I be looking for?
Shawn.
When I first hooked up the DC300A for test I did not have the volume pots connected. As it turns out, the DC offset set up and procedures rely on these pots to set it correctly as the volume controls are actually part of the offset circuit. Things seem to be running even better since last tuning it yesterday with the orriginal worn-out pots. I am currently looking for new ones. Can I dismantle a couple of alps stereo pots and set them up as singles? They look modular in construction (stacked)?
I have one last known issue. There is a horrible sound of a cap discharging when I turn the unit off. Any thoughts? What should I be looking for?
Shawn.
Hi Shawn,
You are welcome by.
That was normal for this model as I recall. You are talking about a modification, and that's fine.
-Chris
You are welcome by.
Make sure your error amp (op amp) remains running after the main supply quits. Also, look at the doubler supply to see if it's decay coincides with your offset.
That was normal for this model as I recall. You are talking about a modification, and that's fine.
-Chris