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-   -   HDAM with Diamond Buffer - Oscillation (noises) (https://www.diyaudio.com/forums/digital-line-level/319671-hdam-diamond-buffer-oscillation-noises.html)

Blackfear 5th March 2018 08:52 AM

HDAM with Diamond Buffer - Oscillation (noises)
 
2 Attachment(s)
Hello, friend of mine suggested me to try this HDAM with its own out diamond buffer, components should be smd, but i tried to make it from discrete parts, input tranz. is 2SK389, instead BC85x,84x i used BC55x,54x, and for output tranz. i used the same as on the schema (I wanted to know how it sound before smd pcb design). But after soldering and connection it seems that it oscillates into diamond circuit, i get some crackling noises from output (some RF noises) also without input signal and also after reconnection filter feedback out/in connection R15/C5, sometimes it is working fine (btw nice sound..), but sometimes it noises, it doesnt matter which channel it is, (ones left then right...mostly one of them randomly...) i tried RF filters on AC also on DC side, tried decoupling caps near to the rails to FZT tranz., also on rails on pcb after +/- plug input, i tried second power supply, some parts movements, remove all wire connection onto bottom if possible, strengthen ground...strengthen each soldered connections, tried bigger R13 (33ohm) for better capacity loads...but it is the same.... Do you think that main reason is poor design of PCB or i need ultra low noise power supply and put pcb into some shielding grounded box (marantz makes this on thei HDAMs), i will make news smaller PCB (4x5cm), all parts will be now very close each other and will put it into shield steel box which will be grounded. Also will try dual zdt6790 tranzistor insted two separates FZTxx,
Please advice if these noises/oscillation could be remove by some schema adjustment... before new soldering ( i dont have sensitive oscilloscope to seek these RF noises) or advise how should i place differently the most sensitive parts, see my PCB and schema thanks

chip_mk 6th March 2018 01:04 PM

Looks like stability issue. Try with different values for C1. For test start with some huge value e.g. 1uF, it should stabilize the circuit but will severely limit the bandwidth. From this point gradually decrease C1 until noise (i.e. oscillations) reappears. Pick the minimal value that keeps the circuit stabile with some safety margin.
BTW it is not clear (but important for analysis of the problem) how is the signal fed to input.

Blackfear 7th March 2018 07:08 AM

Quote:

Originally Posted by chip_mk (https://www.diyaudio.com/forums/digital-line-level/319671-hdam-diamond-buffer-oscillation-noises-post5362790.html#post5362790)
Looks like stability issue. Try with different values for C1. For test start with some huge value e.g. 1uF, it should stabilize the circuit but will severely limit the bandwidth. From this point gradually decrease C1 until noise (i.e. oscillations) reappears. Pick the minimal value that keeps the circuit stabile with some safety margin.
BTW it is not clear (but important for analysis of the problem) how is the signal fed to input.

Hello, thanks for your idea, i will try it and let you know, what do you mean "how is signal fed" , do you mean which cables/routes are used for input of HDAM through R16, R25 isolation...
As i have informed, stability/oscillation issue appears even withou input signal. Thanks...
I used normal 2 core cord cable...

chip_mk 7th March 2018 11:12 AM

To explain a bit... C1=1uF is definitely overkill, but should provide clear answer is it stability issue or something else. However, don't be surprized if reproduction is awfull in this case.

With this circuit, amount of feedback depends on impedance of connected signal source. Stability margin will be lower with disconnected inputs.

Blackfear 7th March 2018 11:34 AM

Quote:

Originally Posted by chip_mk (https://www.diyaudio.com/forums/digital-line-level/319671-hdam-diamond-buffer-oscillation-noises-post5363832.html#post5363832)
To explain a bit... C1=1uF is definitely overkill, but should provide clear answer is it stability issue or something else. However, don't be surprized if reproduction is awfull in this case.

With this circuit, amount of feedback depends on impedance of connected signal source. Stability margin will be lower with disconnected inputs.

Hello, yea I see now...it is directly connected onto output from ES9038q2m chip, i am reworking PCB now, and has also shielding box, then i will see if it will help if not i will add bigger capacity C1, also has an idea to connect it onto 2x batteries 9V ...

ApolloZhang 22nd April 2019 04:30 PM

Was looking into HDAM recently, due to its similarity to diamond buffers. But AC Analysis shows that this topology has a pole some where above 50Mhz and need proper compensations to deal with... And the bandwidth is also worse than a normal diamond buffer.

abraxalito 23rd April 2019 12:55 AM

The output pair look to me to be rather under-biassed (I've not simmed, just from eyeballing) and the stability issue might be related to the bootstrapping of the collectors of the input pair of the diamond from these. I normally use resistors in series with the emitters of the input pair to set the output pair bias current.

Shinja 23rd April 2019 06:29 AM

R2 and C1 forms zero at 16MHz and it might affect stability. The transition frequency seems to be 5-8MHz. If the output stage has peaking it may let the amplifier go oscillation. Try to connect C1 to the left side of R2.

EUVL 23rd April 2019 07:39 AM

1 Attachment(s)
I placed C1 at a slightly different location.
No access to Gnd on PCB of an opamp.


Patrick

ApolloZhang 23rd April 2019 08:48 AM

1 Attachment(s)
Attachment 751592

Occured to me that a Cn, as in the attachment, could compensate a pole in the right plane without losing much phase margin. About 200p will suffice.


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