tropicalrips 0 Report post Posted December 8, 2004 (edited) i think i got the resistor values crossed up 1+(R2/R3) = gain, this is only for 1 side rite? the other side is 1+(R6/R7) correct jason? can explain what is R1,R5 for? output resistor? and also R4,R8? i tried to trace the board, but its a little confusing with each opamp buffering itself i think i should be able to desolder the resistor the resistor values that i hav now on the board : R4,R8 : 100k R3,R7 : 1.5k R2,R6 : 42r R1,R5 : 42r jason, what is the result of my resistor values? is this whats causing the problems? the amp still works but not with opa2132 with these values myR9,R10 is 4.7k , is this whats causing the ground opamp to heat up? Edited December 8, 2004 by tropicalrips Share this post Link to post Share on other sites
tropicalrips 0 Report post Posted December 8, 2004 i managed to desolder the resistors and changed the values following those given by jason the heat problem is not as bad now, opa2132 now works on the board but it sounds different from before the rework...i actually prefer the sound of the amp with the wrong values will need jason the analyse what "went wrong" with the values i first use on the board... my gain now is 6 R1,R5 = 42r R2,R6 = 100k R3,R7 = 20k R4,R8 = 20k R9,R10 = 4.7k Share this post Link to post Share on other sites
bpribadi 0 Report post Posted December 9, 2004 The gain is (1+ R2/R3)= 4 If you got it wrong I still can spare you one more board. I feel uneasy if this proto don't work and I call for production. It will be a very costly move. For the ground channel, a few configuration. 1) OPA2132BP, the 2 output of this opamp in this single IC is short together. The chip will get warm but it gives the highest output current theoritically. Problem is it may oscillate and hear heavy distortion. It will get very very hot. Try option 3 if it doesn't work. 2) OPA134, this single opamp IC can be use with this board by bending the IC leg. Cut away pin 1, 5 and 8. Bend pin 6 to pin 7 location and pin 7 to pin 8 location. 3) Adding a resistor between the output pins with opa2134. Under the board, you can see thick traces short between 2 & 7. Cut them and join a 10 ohm resistor underneath. Use this config if option 1 causing distortion and heat up. Well Jason, I don't know what your reason to stick with the OPA2134 or 132 for the ground channel, to me it is just doesn't seem right in your amp Option 1 is not good theoritically. We cannot just short outputs of op-amps without any resistors. With resistor, we already discussed that it is bad for ground channel. Option 2, are you sure you want to use bended pin Op-Amp for your production amp? Come on... So, the best option for ground channel is to use single op-amp with high current (LM6171, OPA551) in unity gain config. For the gain, I think 4 is too low. Well what I can see from my 'marketing' point of view, the one who need budget headphone amp is the one using high impedance headphone. I don't think those who use 32 ohm headphone will be interested with budget headphone amp. And for high impedance headphone, for example Beyer DT-880, 250 ohm, even gain 6.6 is not loud enough when connected to PCDP line out. And the 600 ohm will need even higher gain. So I suggest you set the gain to around 8.5 - 11. Also, why do you need A47 circuit (high current circuit) for gain 4? Gain 4 means the ouput voltage is low, therefore the current is also low. A simple CMoy will be just nice for gain 4. A47 is needed when the output voltage is high, therefore you need high current output. And also if the output resistor is too high, like 47 ohm, there will be too much voltage drop on the output resistor, waisting the limited low voltage supply from the battery. I tested 12 ohm, and it work perfectly. Sorry Jason, I don't mean to question your design... I just hope you will finaly make a good production amp Why should I wrote so many if I don't care about it Share this post Link to post Share on other sites
av98m 0 Report post Posted December 9, 2004 Actually Bram, I think the target market for this budget amp will be using low impedance portable earbuds/earclip-ons/headphones with 32ohms impedance, with their portable cd or mp3 players. My own 0.02 cents: Why don't you guys toss out the design and just stick to the original chu moy schematic? At least we know that's sure to work, without any more need for R&D. Share this post Link to post Share on other sites
bpribadi 0 Report post Posted December 9, 2004 Well, for 32 / 16 ohm headphone user, I think A47 with gain 4 is overkill. I agree with av98m, the simple CMoy is good enough for it Anyway, the Jason A47 circuit is working fine, only the ground channel need modification Share this post Link to post Share on other sites
jtfoo 0 Report post Posted December 9, 2004 To minimise voltage drop after the output resistors, you can experiment with 0.3ohms first. I saw this on a application notes from national. Share this post Link to post Share on other sites
jasonhanjk 0 Report post Posted December 9, 2004 Where got sell LM6171? I like to try out with this circuit. Bram did manage to talk some sense, this amp is for newbie, it must be dirt easy to install. Tropical, yes. R1 = R5. R1 is at the output of the opamp, or so called output resistor. R4 is for frequency response. Increasing it to 100k will move the DC cutoff from 4.8Hz to 1.59Hz. Your low frequency response would be much better and lower distortion on the low side. I made R3 and R4 equal so that the DC offset is minimal. Analyzing tropical earlier resistor value..... The gain is 1.03. DC cutoff is at 1.59Hz. Maybe you try changing the R4 and you'll get your dream sound.... I admit it, we are just too hardcore in search of finer things in life.... Share this post Link to post Share on other sites
tropicalrips 0 Report post Posted December 9, 2004 To minimise voltage drop after the output resistors, you can experiment with 0.3ohms first. I saw this on a application notes from national. i hav tried 47ohm on output of my amps, its fine for lower impedance headphones (grado) but with Senns they sound muffered, which is why i use 42r for the new board. most of my other DIY amps do not hav the output resistor, at gain of 6 it still sounds ok with lower impedance cans, but if increase gain to 8 or higher, can hear noise, sound like radio or static Share this post Link to post Share on other sites
tropicalrips 0 Report post Posted December 9, 2004 Analyzing tropical earlier resistor value..... The gain is 1.03. DC cutoff is at 1.59Hz. Maybe you try changing the R4 and you'll get your dream sound.... I admit it, we are just too hardcore in search of finer things in life.... but it did not sound like the gain was only 1.03...and it was close to my dream sound my DT331 sounded very good on those "wrong values" what about the heat problem? even with the correct values, the opamps still heats up more then my Cmoy & A47... is it too late to change the design? go with the more conventional A47 design...using 2nd opamp to buffer the first, instead of the 2 opamps buffering itself on the 2nd channel, i think the opamps buffering itself may be whats causing the heat & osillation Share this post Link to post Share on other sites
jtfoo 0 Report post Posted December 9, 2004 most of my other DIY amps do not hav the output resistor, at gain of 6 it still sounds ok with lower impedance cans, but if increase gain to 8 or higher, can hear noise, sound like radio or static Those output resistors I mentioned are not for the same purpose as cmoy which is to lower noise when gain setting is too high. These output resistors I mentioned are used when you parallel two or more opamps to stabilse it. Without output resistors, the opamps will fight each other and possibly burnt out. Which explain why some opamps get too hotin your circuit. The Ro1 and Ro2 on firefox circuit is the type of output resistor I'm referring, not cmoy type.. Share this post Link to post Share on other sites
bpribadi 0 Report post Posted December 10, 2004 Where got sell LM6171? I like to try out with this circuit. Farnell has it. Try the search Or can get some sample from National Share this post Link to post Share on other sites
jasonhanjk 0 Report post Posted December 12, 2004 $8 plus. Share this post Link to post Share on other sites
bpribadi 0 Report post Posted December 12, 2004 No, around S$ 5.- only. I bought one from Farnell Share this post Link to post Share on other sites
tropicalrips 0 Report post Posted December 14, 2004 (edited) finally finish mounting the ver 2.0 board in a case, but this time i wanted to a desktop amp for my pc found this nice little case in SLS, has just enough room to house the transformer, regulated power PCB & the amp itself, but its a tight fit though back view here u can see how tight it is to fit everything into the case, the case is aluminium hope this will help with the heat problem of the amp, still listening to make sure the heat does not affect its performance this case has a slight slope/angle on the front panel, which looks nice, but makes mounting of the board a problem, i had to bent the fins of the heatsink on the PSU board to make room for the board which is also pressing against the tranformer...so far this does not seem to be affecting the amp itself a shot of the amp on my desk together with my main PC cost of the aluminium case : $15 Edited December 14, 2004 by tropicalrips Share this post Link to post Share on other sites
bpribadi 0 Report post Posted December 15, 2004 That's cool Tropical! Your amp built like a tank But I think the transformer orientation better be turned 90 degrees to reduce hum My2Cents. Share this post Link to post Share on other sites
