Post by sadface on Jan 30, 2020 19:22:09 GMT 12
Hi Colin,
If I am understanding you correctly. A low pass filter on the input might be set as low as say 200khz without any negative effect and possibly positive effects on sound quality?
I am getting some odd noises from time to time. I might hear it 2 or 3 times in an evening of 5 hours at the computer. It sounds like a high frequency beeping, a bit like a 90's computer modem but higher pitched. It comes randomly and lasts for maybe 10 seconds before going away. Music output is unaffected.
It appears an issue with the preamp as the noise disappears if I turn off the crossover before the power amps and continues if the crossover is turned back on quickly. It is definitely after the volume control as it is unaffected by the volume control.
My working hypothesis is relay chatter from the protection board. I have removed the protection board from my test rig and I will see if the noise presents itself without the protection board. Hopefully the issue is this simple.
I am still impressed with how nice a sound can be achieved for such a low cost. I am still brewing a parts order from Mouser which will include carbon comp resistors and some extra ceramic caps for the input RC filter.
If not the protection board I am guessing oscillation in the 2nd stage of the preamp.
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Pundit
Post by neilsan on Jan 30, 2020 20:49:40 GMT 12
A Clone Note is a passive volume control made up of a light dependent resistor in series with the signal and a normal resistor of a certain value switched to ground to set the volume. It’s meant to sound better than a normal volume pot. Sure does, Colin. I've tried comparing my CN with my Topping D50S into my Musical Fidelity X-P200 amp, which has 2 inputs. Fed from my Arcam Cd player, although the CN had the feed from the stereo outputs and the 50s from the digital out, so not exactly the same inputs. It was pretty hard to tell any difference.
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Post by sadface on Jan 31, 2020 22:22:27 GMT 12
Well Lads,
Early results are that the issue is with the protection board. Since removal from the system I am yet to hear any funny noises, just sweet, sweet music.
I'll continue testing and see if any noises comes up but all things point towards a redesigned protection pcb to use some different relays.
In the data sheet for the Omrom G5V-1 relays I am using is this paragraph:
"Using the Relay in a circuit where the Relay will be ON continuously for long periods (without switching) can lead to unstable contacts, because the heat generated by the coil itself will affect the insulation, causing a film to develop on the contact surfaces. Be sure to use a fail-safe circuit design that provides protection against contact failure or coil burnout."
I was a bit worried about this paragraph when I settled on these relays. My reading of that is that these relays don't like always on type usage. However one has to start somewhere so that's what I did.
I've been sampling around at what relays other people use in low voltage circuits.
I've settled on Kemet EC2-12NJ DPDT relays which will require an update to my pcb, but does allow extra (unnecessary) protection against a fault as I can wire the two sets of contacts in series. In a fault situation this might just allow the relays to survive as any arc will be spread across both contacts.
I have selected these because I have seen this series used successfully in a relay input selector board. I also cannot find anything in the data sheet that might imply they don't like continuous usage.
Watch this space. Unless something else proves to be the issue, the preamp speaker protection board v2 will appear soon.
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Post by Owen Y on Feb 1, 2020 19:47:39 GMT 12
I recall using those Omron 5V relays to switch inputs on my first linestage - and they degraded the sound significantly (loss of dynamics). I've never used relays since - back to simple silver-plated contact (eg Lorlin) rotary switches ever since. YMMV of course.
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Post by sadface on Feb 1, 2020 22:37:58 GMT 12
Well Bugger,
Things just got less simple.
After many hours of noise free operation since I took the protection out, I was beginning to think I was in the clear. Then I had 2 bursts of the weird noise this evening and the protection board is nowhere in sight.
It might be time for me to invest in a scope and see what is going on in there.
Some interesting characteristics: 1) it is in both channels.
2) it does seem to be effected by the volume control although not in a linear fashion. Volume went up and down with the potentiometer but not to the same degree of increase or decrease I would get with an audio signal. (I turned the signal off immediately upon noise starting)
3) changing to volume setting effected the balance of the noise between the 2 channels.
I am not ruling out a power supply issue right now as the current one is slightly unusual and not entirely proven at scale.
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Post by sadface on Feb 2, 2020 9:23:45 GMT 12
After a nights contemplation I am beginning to think the issue might well lie in the PSU.
My logic here is that the issue is in both channels and only the PSU and the volume pot are connected to both channels.
I think I can rule out the volume pot as it a passive device.
This is pointing me towards some instability in the psu.
Measuring the DC voltage at the supply rails on the top preamp board is rock steady in normal operation.
I will see if I can be quick enough with my multimeter next time I get a noise burst and see if I can see any fluctuations on the supply rails. Otherwise I might have to start poking around the PSU board - my suspicion is that I might find something around the ZTX851/951 BJTs.
My other hypothesis is that it could be a grounding issue however I will let that one lie for now while I cast my gaze at the PSU.
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Post by sadface on Feb 2, 2020 19:44:02 GMT 12
G'day Chaps, Another update. I've decided its time to semi finalise this project and place it on the back burner for the time being. The preamp itself sounds fantastic and whilst it can probably be improved upon, that is not too important right now. For this reason I will stick with the current Mark III boards (no coupling caps and the RC filter on the input). Until proven otherwise, I am going to assume that the issue is the power supply. To fix this I will recycle the Jaycar PSU board from the Mark I since it is known to work flawlessly. I have also discovered that I don't need the protection board at all. My primary reason for its use was for AC detection to prevent turn on/off noises; clicks, pops thumps etc. Using the LME49720 I experience no on/off noises to speak of and thus I don't need the extra part. DC protection would normally be implemented at the power amp stage anyway so far as I can tell. With this all in mind, after the ritual domestic duties (Manly stuff like the lawns and laundry) I got to work on the proper chassis (the one the Mark I was mounted in). Many new holes drilled in the floor plate(thank goodness for cutting oil). I've re-orientated the old power supply board and drilled some extra holes to mount the exotic PSU for when I get to the bottom of the suspected instability issue. I've drilled holes for the new preamp boards and also for an L bracket to mount the volume control nice and far back in the chassis with an extension shaft to minimise wire runs.
The rest of the afternoon was spent preparing and staining up a new front panel.
I used some surplus Okume veneer eucalyptus core 12mm plywood that was surplus from the desk I made a few years back.
I am not sure what happened to the power switch socket but it somehow ended up skewed a degree or 2 off vertical.
I used some Briwax brand Danish Oil (https://www.bunnings.co.nz/briwax-danish-oil-1l_p0526629) which I quite like. It seems to be something like a traditional danish oil (whatever that means) and smells much like wax. Cabots brand Danish oil for example smells much more like Polyurethane and doesn't behave as nicely.
I like to use a saturation method of staining whereby I apply as many 'coats' as I can in a sitting by slathering the stuff on until it stops penetrating. I reapply every 10minutes or so until a coat stays 'wet'. Then I wipe off the excess and let it dry properly overnight.
This time, I got in about 7 or 8 coats before it stayed wet. With my desk I repeated this process for a couple of days with a light sand before each subsequent coat for a total of 12 or so coats.
After this comes the Cabots brand Danish oil. I find that the (seemingly) polyurethane based Cabots danish oil will penetrate after the Briwax stuff has stopped penetrating. On my desk I got 3 coats of Cabots on top of the 12 coats of Briwax before it saturated and stopped penetrating.
Using the cabots on top of the Briwax gives a great effect. The Briwax makes the colour rich and deep and while the Cabots on top it gives it sparkle that just pops.
I then gave it a final coat of Cabots clear PU semi gloss vanish for protection and durability but I don't think I will do that for this front panel as its not going to receive the same wear and tear as my desk. Cabots semi gloss PU varnish is not easy stuff to use, getting even coats was a nightmare that I never quite achieved. I suspect it needs to be thinned with clear spirits to behave nicely.
For comparison here is the front panel on my desk. The desk has a slightly different colour as I mixed a colour tint into the Briwax danish oil for the couple of coats, I can't remember what the colour tint was. It turned out slightly problematic in that it didn't coat evenly, its somewhat noticeable from the right angle with an uncluttered desk that there are splotches that were more tinted than the rest of the desk top.
Tomorrow I will slather on some Cabots danish oil and by Tuesday I should be ready for 'final' assembly.
All this will free up my test jig so that I can test the power amp side of the Boombox from my other thread. I have had the PSU and LM1875 boards looking at me for several weeks now begging for some juice.
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Post by colinf on Feb 3, 2020 9:09:53 GMT 12
Always good to have an oscilloscope. Try Emona instruments in Melbourne (emona.com.au). Some opamps don’t like having the RF filter cap directly connected to an input pin and might oscillate at the slightest bit of RF excitation. The combination of the input pin capacitance and the cap intended for RF filtering can form a high-Q resonant peak at frequencies higher than a normal bandwidth oscilloscope can show easily. You just need to dampen the Q of that resonance with a small resistor less than 100 ohms in series with the input pin. I do that on most opamp circuits. Recent opamps have been designed to resist RF with the growing mobile phone, wifi and now 5G RF pollution.
AMR-iFi R&D
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Post by sadface on Feb 3, 2020 20:25:42 GMT 12
Hi Colin, The scopes at Emona Instruments all look a bit pricey. I am going to see if I can pick up something second hand on trade me. Right now there is a some good name brand older scopes. I'll be keeping an eagle eye to see what these are going for. www.trademe.co.nz/Browse/Listing.aspx?id=2517259265 a Tek 475 sitting at $190 for now. www.trademe.co.nz/Browse/Listing.aspx?id=2515693559 This looks like its probably going to go up in price a lot but I might just get lucky. The cheap end of new scopes seems to be all low bandwidth/low bitrate types. I am working on the basis that an older bench scope in good condition with 50-100mhz will end up being superior and more useful in the long run. These noise bursts are doing my head in a tiny bit. I haven't had one since Saturday night but I am sure it will rear its ugly head again soon.
It might just be the catalyst for me to get my 6SN7 Aikido preamp going again. Nice simple tubes.....
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Post by sadface on Feb 28, 2020 21:12:59 GMT 12
Alright chaps, a long overdue update.
The backup preamp is now completed in mark III form.
I ended up sticking with the exotic ultra low noise power supply instead of the original Jaycar one as I had suggested in a recent post.
I never got to the bottom of the strange noise issue. It has not shown its face since everything was mounted in the proper steel chassis.
My working hypothesis is that the noise was a result of external interference from which it is now appropriately shielded. Eventually I will get that oscilloscope so that I can test with something other than a DMM or my ears.
Any further upgrades will be down the track as the performance is now more than satisfactory for its purpose as a reliable solid state backup preamp. I do like the idea of forcing class A however I have some other projects that need priority right now as this requires a redesign of my boards.
I need to make a prototype tube preamp so that I can test out the prototype LM1875 power amp sitting on my bench. It might be OCD, fanboyism or audiofoolery, but I simply can't stomach the idea of not having tubes somewhere in my amp chain. Right now I've got tubes at the power amp end to keep me satisfied.
Until I get itchy for upgrades, may I present the finished product. You can probably tell I am no photographer. I really love the grain of the Okume
OmA nice shot of the finished front panel. Not my best woodwork but passable.
From the front.
Internal shots.
A closeup of the guts.
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Post by colinf on Feb 29, 2020 6:43:24 GMT 12
Looking good! The noises you were getting possibly were RF interference of some kind. RF is so pervasive these days and any trick you can pull to reduce it in audio circuitry is good!
AMR-iFi R&D
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Post by Owen Y on Feb 29, 2020 13:16:41 GMT 12
I rather like the plywood faceplate.
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Post by sadface on Feb 29, 2020 20:17:32 GMT 12
Thanks guys,
I always have a preference for nicely stained wood.
A friend of mine requires an integrated amp with a tube preamp stage and a solid state power amp stage. My evil plan right now is to acquire a cheap/salvage home theater receiver to use as the chassis. The idea being a standard steel box for magnetic/RF shielding and then clad the whole thing in some nice wood for aesthetics.
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Post by colinf on Feb 29, 2020 20:38:10 GMT 12
Good idea. You could re-purpose some of the poweramp channels in the receiver, along with the power transformer. You’d need to add a high voltage transformer for the HT for the valves.
AMR-iFi R&D
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Post by sadface on May 29, 2020 23:51:50 GMT 12
G'day Guys,
May I present version 3.0
I removed the coupling caps as this one is designed to take advantage of the LME49720. I added a footprint for the RC filter to the input. I separated the power ground from the signal ground by running some extra traces on the top side for the power ground. The star is located at the PSU ground. Lastly I added footprints underneath the opamp to accommodate the two E562 diodes.
Any critiques?
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Post by colinf on May 30, 2020 19:34:01 GMT 12
C3 and C- are very close to the ends of the opamp, it will be fiddly to get it out of a socket. Ground planes can have the opamp power supply caps connected straight to it. C2 on the circuit diagram should be 330p or less, and on the board it’s written as 330p. Also on the schematic you need to indicate the connection between the two grounds. With 330p it’s good to drive it from low impedance to maintain frequency response. Otherwise looking good, the constant current diodes will bias the output stage into class A, and no coupling caps will lift SQ.
AMR-iFi R&D
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Post by RdM on May 31, 2020 17:26:20 GMT 12
No pictures showing ... www.isitdownrightnow.com/ibb.co.html Response Time: no response Down For: more than a week DOWN Ibb.co is DOWN for everyone. It is not just you. The server is not responding... Yet I see people have posted as though they can see the pics in less than a week ago. Is it really just me after all? ;=}) (Or maybe they have cached images in their browser? First time I read this thread, just now.)
UPDATE: an hour or so later. All images working again. Must have been a temporary outage. isitdownrightnow or similar websites are useful resources though, to check such a situation.
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Post by sadface on Jun 28, 2020 18:36:17 GMT 12
'Ello Chaps,
While waiting for parts for my current gainclone project I decided it was time to work on Mark IV. Class A time!
The boards are soldered up and ready for installation. The E562 constant current diodes are mounted on the underside. I probably could have mounted them topside under the socket but oh well.
Here is the Mark IV board next to the Mark III boards for a size comparison. Can anybody spot the stuffing mistake? It probably should have keep the old hole spacing as now I have to drill some extra holes in the chassis.....
I will recycle the same LME49720 chips since these have already been burnt in.
All going to plan I will have this up for testing tomorrow.
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Post by sadface on Jun 30, 2020 20:44:21 GMT 12
G'day Boys,
Mark IV has passed initial testing! No sparks or magic smoke on startup.
I tested it on the boombox and its works perfectly.No strange noises, hiss or hum. Just Radio concert New Zealand playing sweetly.
Tomorrow I will take the 12VAC Aikido prototype out of my main rig and plug in the Mark IV for some proper testing at volume.
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Post by colinf on Jul 2, 2020 19:32:25 GMT 12
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Post by sadface on Jul 9, 2020 18:24:57 GMT 12
G'day Guys,
I've been listening to it for a week or 2 now and the results are pretty spectacular. Unfortunately I cannot compared it directly to the Mark III because the Mark III was out of my system for too long.
Compared to the 12vac Aikido there is improved detail, especially in the trebble. Deeper bass due to the lack of a high pass filter on the output. It hasn't presented itself as fatiguing which is a bloody good sign. I am typically intolerant of solid state preamps in longer listening sessions.
Of course, what it doesn't have is tube warmth. I obviously enjoy the sound of distortion because I find myself missing the Aikido at times.
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Post by colinf on Jul 10, 2020 5:50:24 GMT 12
Glad it’s working well. Missing the valves...that’s the ol’ valve vs SS thing. Nelson Pass has been making inroads into that and concluded that adding controlled amounts of harmonic distortion to SS results in a more satisfying sound than straight low-distortion amplifier systems.
AMR-iFi R&D
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Post by sadface on Jul 10, 2020 8:39:52 GMT 12
Hi Colin,
I agree, it does just come down to the valves vs SS argument.
For a preamp, I am firmly in the valves camp.
That being said, for the cost and simplicity. This preamp is pretty darn nice. It should give me a long and admirable service life as it’s name suggests, as a backup.
Soon I will introduce my next big project. A full sized, dual mono aikido preamp.
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Post by colinf on Jul 11, 2020 20:27:32 GMT 12
Valve vs SS....an example is the iFi Pro iDSD dac/streamer which uses both a jfet class A output stage or a GE 5670 valve output stage with 2 modes. The first mode has low distortion, the second allows a bit more of the natural tube harmonics through. So it’s quite flexible sonically with regards to the specific setup and taste.
AMR-iFi R&D
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