While I've read many interesting threads in the search for my oscillation problem (BrianGT chipamp, single 37VDC supply), I haven't seen much mention or use of the optional L/R parallel output filter as described in the National tech notes. In fact, this design is the first one I can recall which uses the optional filter on the output.
Most designs I've seen, including the one I am using, include the R-C Zobel. But why not the L/R filter? Is it simply to reduce component count for something probably not needed? Might it help solve oscillation where the pre-amp feeding the amp is introducing the oscillation somehow? I'm still trying to understand this thing a bit better.
I know now that my NAD 3020 preamp causes my amp to oscillate. It could well be a mod that I did on it (and I'm thinking of reverting back to stock to find out). But I would also like to understand this better. My house is probably full of RF (at least 4 PCs on at any time, lots of network gear, 2.4G wireless, cellular, etc.), but I really don't know if this is a factor or not.
I do like the sound of this thing - I've been listening to it most of the day on a PC sound card playing the Artur Rubinstein collection, and it sounds quite good... but I wish I could hook it to my main system without it melting down from oscillation! Perhaps I should just get a selector and volume pot, and skip the pre-amp...
Any thoughts on the filter are welcome
Most designs I've seen, including the one I am using, include the R-C Zobel. But why not the L/R filter? Is it simply to reduce component count for something probably not needed? Might it help solve oscillation where the pre-amp feeding the amp is introducing the oscillation somehow? I'm still trying to understand this thing a bit better.
I know now that my NAD 3020 preamp causes my amp to oscillate. It could well be a mod that I did on it (and I'm thinking of reverting back to stock to find out). But I would also like to understand this better. My house is probably full of RF (at least 4 PCs on at any time, lots of network gear, 2.4G wireless, cellular, etc.), but I really don't know if this is a factor or not.
I do like the sound of this thing - I've been listening to it most of the day on a PC sound card playing the Artur Rubinstein collection, and it sounds quite good... but I wish I could hook it to my main system without it melting down from oscillation! Perhaps I should just get a selector and volume pot, and skip the pre-amp...
Any thoughts on the filter are welcome
In my case, the L||R filter is there to ensure that the amp doesn't oscillate when I connect electrostatic (or any other speakers) speakers. That amp has been in daily use with my ESL-63s and some ESLs of my own construction for about 5 years now. It has never "complained".
I learned a long time ago that a working amp with an L||R filter sounds a lot better than a dead amp and/or speakers due to amplifier oscillations. Why don't others use it? I guess some people have to learn that the hard way.
I_F
I learned a long time ago that a working amp with an L||R filter sounds a lot better than a dead amp and/or speakers due to amplifier oscillations. Why don't others use it? I guess some people have to learn that the hard way.
I_F
I used it in all my LM3886/LM3876 amplifiers. I have also seen it in published circuits. But this was at least ten years ago, before the internet discovered the LM and the hype was founded.
Perhaps it has something to do with the gain clon(ing). Someone starts with a specific design and others just copy it.
juergenk said:
Thanks for this info. Tonight, I found a 2W 10ohm resistor, and wound a coil using 24ga cat5 wire around a 1/4" drill bit, with 15 tight turns. ran the resistor through the core, then added to one amp speaker output.
No luck. :-(
With my 3020 preamp back in the system, the amp with inductor still climbs from 20C to 40C at the heatsink within 4 minutes. With no other change except swapping source back to a PC sound card, the temp drops to 35C within a few minutes, and will stay below 40C for hours.
I can't understand what the 3020 is doing to cause such problems. Perhaps I should just start reversing all the mods I did on it... (basically all the pre-amp mods here, except bypassing balance/mute)
I also have only replaced a few of the dozen or so caps on the regulated PS board for the pre-amp, and have read this board is sensitive to changes in values... I wonder if this could be it? Voltages are a bit higher than spec at +31.5/-26.5 compared with +29.9/-25.6)
I think I'll undo the mods, since I wasn't sure I could hear any change anyway...
Eva said:
You are absolutely correct! I just finished un-modding my 3020 back to stock, and the first power up confirmed this - the power on/off 'click' is gone, and temps are now stable at ~32C at low levels. Excellent!
It is interesting that the 3020 power amp section handled the HF fine, presumably the design is 'safer' to deal with problems like this, although I doubt they anticipated newbies like me messing with BEE's design 30 years later... ;-)
Thanks to all for the info on this little wild goose chase - now I can get back to finishing the amp. I am considering adding a pot, and a source selector switch for 4-6 inputs. Any tips on sources for these at decent prices? I can't seem to find suitable stuff at digikey, but I'm probably not looking in the right places...
TDWesty said:
You are absolutely correct! I just finished un-modding my 3020 back to stock, and the first power up confirmed this - the power on/off 'click' is gone, and temps are now stable at ~32C at low levels. Excellent!
That's the problem with getting modification info off the web - you don't know if the person doing the mod and telling you how to do it actually knows what they are talking about. It becomes a case of the blind leading the blind. You are lucky that it didn't destroy your amp and/or speakers (this time).
I've never understood this "mod culture" stuff. It is one thing for engineers to look at a circuit they understand and try to figure out how to make it better, but where do people who know little or nothing about circuits get the idea that they can start clipping out "extra" parts and swapping op-amps and end up with something better? The mind boggles...! I wonder if people do the same thing with their cars, motorcycles, cameras, etc.
I_F
forr said:
I'd ask what you think is wrong, but first, please fill in the following questionaire so I can determine whether the advice is worth your effort to type it....
Answer True or False:
1) My cables are more expensive than the rest of my audio system.
2) I regularly demagnetize my CDs to improve their sound.
3) The green pebbles seem to have a better effect on my system's sound than the brown and black ones.
4) I wish Stereophile would bring back Jonathan Scull. He was the best reviewer in the history of that magazine.
5) I can hear the difference when I reverse a resistor in my amplifier/preamplifier.
6) My knowledge of audio electronics is exceeded only by my knowledge of fine wines, luxury automobiles, and the Opera.
Please submit the answers to this questionaire at your earliest convenience and I will have my seconds get back to you...
Thank you.
I_F
I_Forgot said:
In my case, I was trying to simplify a bit, and remove the tone/balance/mute controls, which I never use, but, as you state, doing even a simple mod without understanding the design has risks.
My Dad's 7020 had the tone controls bypassed by the NAD dealer when he bought it new in the 80s, so I was hoping to emulate this change. Oh well. I will revert to simply restoring it to original specs by replacing the remaining caps, and leave it alone.
Funny you mention cars... I also drive a "modified" van, but I understand the design of the drivetrain a *lot* more than these amps! (it's a VW Westfalia with a Jetta TD engine - doing my part to reduce greenhouse gases...
Hi_Forgot
I.
1,2,3,4,5 : I hope for you that all are false.
6. I can't know. I can only speak for myself, I live in a wine country, my knowledge of audio electronics is on the par with my knwledge of fine wines.
II
. As any voltage difference, resistively, inductively or capacitively induced, between the input ground and the feedback ground will be amplified, input and feedback must share the same ground and not have separate ways to return to the star connection.
. See Douglas Self for correct reservoir capacitors wiring. Circulating rectifying current pulses must not be shared with other parts of the circuit, which means stay away from the star point.
The drawing has been done with an obsessive idea of star grounding in mind, which shows how the concept can be misleading.
I.
1,2,3,4,5 : I hope for you that all are false.
6. I can't know. I can only speak for myself, I live in a wine country, my knowledge of audio electronics is on the par with my knwledge of fine wines.
II
. As any voltage difference, resistively, inductively or capacitively induced, between the input ground and the feedback ground will be amplified, input and feedback must share the same ground and not have separate ways to return to the star connection.
. See Douglas Self for correct reservoir capacitors wiring. Circulating rectifying current pulses must not be shared with other parts of the circuit, which means stay away from the star point.
The drawing has been done with an obsessive idea of star grounding in mind, which shows how the concept can be misleading.
forr said:
OK, I'll give you a waiver for the wine. You pass.
I'll look at the grounding at the input and feedback again. As built the amp is completely quiet, so I don't think I can make any improvement, but it is worth investigating.
The PS ground (the two green wires in the upper terminal block) goes to the star ground. I seem to recall reading that if a single bridge rectifier is used, the center tap of the transformer should be grounded to the chassis along with the capacitors at a point remote from the audio star ground, but with a dual rectifier the ground should be made at the audio ground because the rectifier currents are restricted to the wires between the caps and the rectifiers. I will make some drawings and think about it more. You may be right.
I_F
Hi Tdw,
Do not connect anything directly to chassis except the safety ground.
Then make the rest of the circuit safe by whatever method does not put the users at risk.
Does the output of the preamp measure 0Vdc? and with the input shorted 0Vac? Those supply rails have a surprising offset! Why?
Does you chipamplifier have a low pass and high pass input filter?
BTW,
I favour the full Thiele network (R+C & L//R) rather than just the Zobel (R+C) for the output.
It helps to amplifier maintain the same stability margin irrespective of the quirky loadings that can be hung on the end.
The Thiele network will have a down side.
The amp will not respond to cable tuning as readily.
this paragraph could be completely misleading if improperly applied to a circuit that the originator did not have in mind.
Do not connect anything directly to chassis except the safety ground.
Then make the rest of the circuit safe by whatever method does not put the users at risk.
Does the output of the preamp measure 0Vdc? and with the input shorted 0Vac? Those supply rails have a surprising offset! Why?
Does you chipamplifier have a low pass and high pass input filter?
BTW,
I favour the full Thiele network (R+C & L//R) rather than just the Zobel (R+C) for the output.
It helps to amplifier maintain the same stability margin irrespective of the quirky loadings that can be hung on the end.
The Thiele network will have a down side.
The amp will not respond to cable tuning as readily.
AndrewT said:
I think we're talking about the schematic I posted in the opening post, not the briangt chipamp I am actually using. Or are we talking about a third option...
As it happens, I am using a single bridge design with CT transformer, so this advice does apply to me I think. My grounds are mounted to a screw post on the metal chassis in this order:
1. G from AC line input
2. CT from transformer
3. PS board ground with a .1uF/10ohm (parallel) isolating it from the ground point (taken from other advice I read here...)
4. separate leads from each chipamp board designated as CHG chassis ground.
The amp input and output grounds are not attached to ground except as designed on the board (ie: return through the amp board CHG lead).
I get very little noise when connected to the fixed 3020 preamp, but a fair bit of hum/hiss when connected to the PC sound card.
Yes, the pre out is 0VDC now (after I undid the mods). There is a cap to remove the 2.5VDC offset, and one of the mods I had done removed this cap and used a resistor voltage divider to reduce the offset to 0. I believe it was 0VDC with the mod, but don't actually remember for certain.
The chipamp does not have any input filters that I can determine. My heat/oscillation problems were definitely caused by the 3020 mods, but I am still interested in making the amp more stable and tolerant of such err... abuse.
Although adding my home made L//R didn't help, I will probably try to include this in the final design to be safe. Thanks for the info.
Hi,
Where is the PSU 0v (=capacitor common) connected?
check that you have at least the RF filter in place. It is usual to set it between 1.5uS (high treble very muted) to 0.3uS (no discerible treble cut but lets more RF through).
where is the speaker return connected? Can you temporaily remove all the audio grounds from this post but leave the safety ground intact (it should be permanent)?
Where is the PSU 0v (=capacitor common) connected?
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