what's the input impedance of three parallel connected inverting 3886?
What is the lowest frequency you need for guitar?
These look good. 19" Gehäuse Rack 2 HE schwarz mit Griffe und Füße Stahl on eBay (end time 17-Apr-10 00:16:15 BST)
Really cheap 19" rack cases from Germany
Really cheap 19" rack cases from Germany
Yup, that 19" rack is a good deal. It must be flat-packed before shipping, right? There's no way it coild be shipped worldwide for 17 euro otherwise.
The only issue is with all the fine print - I can't understand krautspeak, so it has to go into an online translator, where the nuances may be lost.
The only issue is with all the fine print - I can't understand krautspeak, so it has to go into an online translator, where the nuances may be lost.
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Yes it is. Iv'e ordered one so I can let you know. I think the radiospares was better because the front/rear plates were aluminium but by time they added the vat and postage it was about £65 so I decided to try the German one. I like the black finish also. So you have bought another Goldmund/SymAsym. It will be interesting to compare the first modded one with the second with the other mods. You must let me know how you get on with it. I can't wait to get mine in the case. I still haven't got a pre-amp yet, but there is no hurry. I'll wait till the cash flow is better.
Hi Linuxguru, Problems with the feedback resistors! I'm a bit thick. Can you how many and where they are located. I thought there were two of each, but iv'e found four. I'm not sure which ones. help appreciated. Barry
There are two per channel, 22k and 1k, located near the 220uF/50V Panasonic FM DC-blocking cap in the NFB network (fairly close to the input connector). All 4 resistors form a tight rectangular cluster next to the 2 capacitors.
Hello,
i just registered here, because this thread seems to be a very good source regarding the Yuanjing PCBs and its modifications. (Thanks especially to linuxguru). Sorry for hijacking and/or ressurecting this thread but i have a question i could not solve on my own for those who already built amps with these boards:
I have a problem with loud "plop" noise after switching on the amp, and an even louder "plop" after switching it off (2 - 3 seconds after power has been cut).
if i measure DC voltage on speaker terminals it is 0,00V while running and it spikes to 5V DC at the time the noise occurs when turning off the amp. This is exactly the same on both channels.
Now i am afraid to ruin a speaker by this large DC spike - is this board defective or what could be the reason for this? Please if anyone could help me that would be very nice.
Thank you!
drjames
i just registered here, because this thread seems to be a very good source regarding the Yuanjing PCBs and its modifications. (Thanks especially to linuxguru). Sorry for hijacking and/or ressurecting this thread but i have a question i could not solve on my own for those who already built amps with these boards:
I have a problem with loud "plop" noise after switching on the amp, and an even louder "plop" after switching it off (2 - 3 seconds after power has been cut).
if i measure DC voltage on speaker terminals it is 0,00V while running and it spikes to 5V DC at the time the noise occurs when turning off the amp. This is exactly the same on both channels.
Now i am afraid to ruin a speaker by this large DC spike - is this board defective or what could be the reason for this? Please if anyone could help me that would be very nice.
Thank you!
drjames
It's par for the course - The turn-on pop can be mitigated a bit by increasing the value of the mute capacitor, but the turn-off pop is a function of the minimum supply voltage of both the op-amp and chipamp, and it is difficult to control. As the rail voltage falls, the DC feedback loop will open at some level for the opamp and/or the chipamp and cause a pop. You can try rolling different opamps to control the turn-off pop - I found that using a low-rail opamp like the LT1213 causes a quieter pop, than say an LT1208.
It's strange because I do not get a pop at all with this amp. Not during power on or during power off. The only difference is that I fitted larger smoothing caps, replaced the op-amp with a LM4562 and fitted a roxburgh mains socket that includes a suppressor circuit.
Hey thanks for the quick replies!
It is very strange - i really think theres something wrong with the board/one of the components or i did something wrong with grounding or something else as this is my first amp project...
The switch-off pop is so hard/loud i am afraid to connect a speaker to this amp until i fixed it - the speaker membranes jump out at least half a centimeter when the pop occurs! this can't be good for the speaker can it?
The PCB is mounted in a metal case (with metal spacers) that is grounded to earth directly to mains socket. So everything is grounded.
But i also tried to run it out of the case with the same results.
here is a pic of my current setup:
one thing that got my attention is that the red+green led both light up when i power it on, but when i turn off, the red led stays on for 10+ seconds while the green one fades out rather quickly (1-2 sec). should'nt they fade out about the same speed because of the symmectric layout?
i hope i don't bother you all to mucht with my (probably stupid) questions 😱
thanks for your time!
drjames
It is very strange - i really think theres something wrong with the board/one of the components or i did something wrong with grounding or something else as this is my first amp project...
The switch-off pop is so hard/loud i am afraid to connect a speaker to this amp until i fixed it - the speaker membranes jump out at least half a centimeter when the pop occurs! this can't be good for the speaker can it?
The PCB is mounted in a metal case (with metal spacers) that is grounded to earth directly to mains socket. So everything is grounded.
But i also tried to run it out of the case with the same results.
here is a pic of my current setup:
one thing that got my attention is that the red+green led both light up when i power it on, but when i turn off, the red led stays on for 10+ seconds while the green one fades out rather quickly (1-2 sec). should'nt they fade out about the same speed because of the symmectric layout?
i hope i don't bother you all to mucht with my (probably stupid) questions 😱
thanks for your time!
drjames
Both LEDs should fade out within a few seconds of each other, but if there's significant DC offset at the output, the LED corresponding to that rail will fade out quicker, because the corresponding capacitor will discharge quicker through the speaker (causing a pop in the process).
Also check the feedback network of the chipamp - it should be 22k and 1k on the assembled Yuanjings, but it appears that several have been erroneously assembled with 10 ohms instead of 1k. That will make the closed-loop gain too high, and could also contribute to high output offsets.
Also check the feedback network of the chipamp - it should be 22k and 1k on the assembled Yuanjings, but it appears that several have been erroneously assembled with 10 ohms instead of 1k. That will make the closed-loop gain too high, and could also contribute to high output offsets.
Hmm,
i am really at a loss. Thanks for the tip with the feedback resistors, but they are indeed 22k / 1k. The thing is, the output dc offset is really really low while running (my cheap multimeter shows 0.00 and 0.01 mV on each channel), but only while turning on/off i get a discharge of around 5 Volt (at least that is what my meter shows, could be even higher peak because of the slow reaction time).
One thing that i should mention is that the amp sounds quite good when i connect speaker to it after turning it on and disconnect before turning off. No hum, no noise etc., it only seems to lack a bit of bass but that could be due to the small transformer 120VA 2x18V (?) or the temporary wiring with thin cables on input side.
I was really thinking about adding a relais board that connects the speakers only after say 2 seconds after turn-on. But otherwise i would really like to know the source of the problem. (Which i still think is a bad board/ bad component) Maybe i should just order a second board from ebay and see if that behaves differently :-(
i am really at a loss. Thanks for the tip with the feedback resistors, but they are indeed 22k / 1k. The thing is, the output dc offset is really really low while running (my cheap multimeter shows 0.00 and 0.01 mV on each channel), but only while turning on/off i get a discharge of around 5 Volt (at least that is what my meter shows, could be even higher peak because of the slow reaction time).
One thing that i should mention is that the amp sounds quite good when i connect speaker to it after turning it on and disconnect before turning off. No hum, no noise etc., it only seems to lack a bit of bass but that could be due to the small transformer 120VA 2x18V (?) or the temporary wiring with thin cables on input side.
I was really thinking about adding a relais board that connects the speakers only after say 2 seconds after turn-on. But otherwise i would really like to know the source of the problem. (Which i still think is a bad board/ bad component) Maybe i should just order a second board from ebay and see if that behaves differently :-(
A DC detection/speaker protection circuit is never a bad idea, and the kits usually have a ~4 second delay before enabling the output relays.
Does the transformer hum, vibrate or get warm? Would be an indication of a shorted cap and you can test it by checking the voltage on the hot leg (+/-) of each cap.
Mains AND op-amp caps
Mains AND op-amp caps
Im not sure about the Impedance but 30-ish hz is the lowest freq. 41hz is low E but 5 and 6 string basses go lower.
find out the input impedance to allow you to check the input filter and find if that suits guitar.
Got my new case for the Goldmund/SymAsym, nearly finished, but it still gets very hot after just a few seconds. Will try to post pics when complete. The Yuanjing is sounding great.
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