hi prasi
so i understand it right from the beginning, i can't get a good offset, the max bias i can get with the pots is about 1.1 v / 110 ma thats ok, but i get these measurements setting the bias to 0.88 v / 88 ma:
channel a
offset 0.457v
over r2(47ohm) 54.9 ma over r3 (47 ohm) 75.7ma
r8 (1k) 5.05v
channel b
offset 0.348v
r2 (47ohm) 60ma r3 (47ohm) 78.3ma
r8 (1k) 5.22v
the offset rise steady with the bias, starts near 0 both pots closed but jumps to about .170v with a low bias at around 10ma. also the mosfet don't get hot, i think there is a desing problem. measured all the resistors and checked all the parts twice, everything is at the right place (and don't smell bad ;-) ). looks like the amp has start problems. any idea?
so i understand it right from the beginning, i can't get a good offset, the max bias i can get with the pots is about 1.1 v / 110 ma thats ok, but i get these measurements setting the bias to 0.88 v / 88 ma:
channel a
offset 0.457v
over r2(47ohm) 54.9 ma over r3 (47 ohm) 75.7ma
r8 (1k) 5.05v
channel b
offset 0.348v
r2 (47ohm) 60ma r3 (47ohm) 78.3ma
r8 (1k) 5.22v
the offset rise steady with the bias, starts near 0 both pots closed but jumps to about .170v with a low bias at around 10ma. also the mosfet don't get hot, i think there is a desing problem. measured all the resistors and checked all the parts twice, everything is at the right place (and don't smell bad ;-) ). looks like the amp has start problems. any idea?
Hi rumina,
Its been tested by atleast two people (xrk971 and e_fortier) that i know of. i think they have tested the mono version and the values were fine tuned (R2/R3) based on xrk's test.
schematic in post #392.
for my info, could you let me know,
1. which transistors are you using bc550/560 or the 547/557?
2. are they matched?
reg
Prasi
Its been tested by atleast two people (xrk971 and e_fortier) that i know of. i think they have tested the mono version and the values were fine tuned (R2/R3) based on xrk's test.
schematic in post #392.
for my info, could you let me know,
1. which transistors are you using bc550/560 or the 547/557?
2. are they matched?
reg
Prasi
In order to measure the DC offset, make sure you short circuit the input signal and do not put any load on the output like a headphone until everything is OK. My bias is 100mA with around 2mV DC offset. Supply is regulated +/-15Vdc. This is with the mono HA, I haven't tried the stereo one with the volume.
I used matched BC550/560. I normally ordered cut-tape, makes it easier to match.
All my components are from Digikey, no fake devices.
For me R2 = R3 = 68 ohm like in the original design.
Keep us posted.
BR
Eric
I used matched BC550/560. I normally ordered cut-tape, makes it easier to match.
All my components are from Digikey, no fake devices.
For me R2 = R3 = 68 ohm like in the original design.
Keep us posted.
BR
Eric
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thanks for the feedback, input is shortend and no output. i first used some matched bc550/bc560 from ebay but changed it to some mouser bc547/bc557 after the probems. the matched one fron ebay where fine, the offset was a bit higher with the mouser parts but only about 10%. the offset is around 200mv with poth pots closed and a bias of 6-8ma, thats strange. it's a littel bit hard to measuer this pcb, it is so thightly packed, maybe you see something when i post it:
Attachments
No. In fact even for single pcb, people built it with 330ohm x 3 or single 1k resistor. 130 ohmx3 just an initial suggestion by AndrewT.
Nicely built, but this pcb isnt designed for vishay RNXX resistors. those are quite thick.
one more thing, are you using 2 multimeters to monitor both Id and offset at the same time as advised by Juma?
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care was needed to put the resistor in but they dont shorten other parts, yes i use two multimeters for bias and offset. strange for me is that both channels have the same problem; what would be the effect when i change r2/r3 to 67ohm?
referring your post #501, measurements mentioned over r2 and r3 are in mV, right?
Then the current through i/p pair becomes 1.17 mA and 1.6mA, which is a bit high, so better to go with 68ohm (I feel), as recommended is 1mA.
Also can you measure delta V across R10 and R11, that gives an idea of current thro second stage (should be 4-5mA)
Then the current through i/p pair becomes 1.17 mA and 1.6mA, which is a bit high, so better to go with 68ohm (I feel), as recommended is 1mA.
Also can you measure delta V across R10 and R11, that gives an idea of current thro second stage (should be 4-5mA)
hi prasi
thanks for helping me, the measurements where ma, with the pots closed i get these results for voltage of stage 1 and 2:
channel a
r2 41.1mv
r3 41.4mv
r10 3.2v
r11 3.465v
channel b
r2 41.7mv
r3 41.4mv
r10 3.208v
r11 3.498v
and thanks for the layout, easier to see than the picture of the pcb. i don't understand why i get a offset of 0.2v when the pots ar closed and the bias is low as 10ma. as soon the pcb get power the offset rise up to 0.2v in a minute or two. i think the first stage is fine, is the seconde stage the trouble maker?
Hi rumina, you are welcome.
If the offset is +0.2VDC, try and see which pot adjustment leads to decrease in offset (when both pots are at say 2.5k or 50%). keep on adjusting the same pot to bring it near to zero while monitoring the Id. this will help us in knowing whether the DC offset can be adjusted at all. its what i feel can be done here. may be Juma could chime in with his views.
if the dc offset doesnt change at all when the pots are adjusted, there could be a problem somewhere.
If the offset is +0.2VDC, try and see which pot adjustment leads to decrease in offset (when both pots are at say 2.5k or 50%). keep on adjusting the same pot to bring it near to zero while monitoring the Id. this will help us in knowing whether the DC offset can be adjusted at all. its what i feel can be done here. may be Juma could chime in with his views.
if the dc offset doesnt change at all when the pots are adjusted, there could be a problem somewhere.
no way to bring it near zero, i can bring it down from 0.2v to 1.7v when one pot is fully open and the other completley closed, bias is then around 35ma. i allways measure the pots befor mounting and set them to 50%. may a 10k pot or a resistor in parallel a solution?
no matter which pot you adjust and in whichever direction (ccw or cw rotation), does the offset always stay in positive volts?
whatever resistor values that i could make out from the photo, are correctly located. you could try with a fresh pair of closely matched i/p bc550/560. atleast a DMM match. better would be transistor tester.?
you are using C1 as 1uf and R5 as 100k, right?
Also i/p shorting should occur at pin no 1 and 2 of the volume potentiometer? here is the schema of left channel.
Attachments
will do that, often better to wait some time to clean the head an start again. any sugggestions may help ;-).
hi prasi
thanks that you take time for me, i don't think i have shorts or cold joints, i always clean the pcb with isoethanol and check the joints. happy when you check the picture for yourself.
i took the chance of a stady hand in the morning and removed the bc547/bc557 and took a octet of matched bc550/bc560 in. i set the pots to 2.5k and get these results:
a
offset 340 mv
r13 127mv
r17 132 mv
b
offset 341mv
r13 150mv
r17 154mv
now with the machted set the measurments for npn and pnp and for a and b amp are similar, so i think the bc550/bc560 are fine. i ordered some irf610/rf9610 and change them tomorrow. also i think the pot could be 10k. the resistors are placed like printed on the pcb, i checked them 3 times now, i don't see a mistake in soldering/placing the parts.
ps the two c3 330pf are missing in the picture, had some trouble once with the one i have and desolderd them today / ordere some new one (don't think they have a great effect on the offset ;-) )
thanks that you take time for me, i don't think i have shorts or cold joints, i always clean the pcb with isoethanol and check the joints. happy when you check the picture for yourself.
i took the chance of a stady hand in the morning and removed the bc547/bc557 and took a octet of matched bc550/bc560 in. i set the pots to 2.5k and get these results:
a
offset 340 mv
r13 127mv
r17 132 mv
b
offset 341mv
r13 150mv
r17 154mv
now with the machted set the measurments for npn and pnp and for a and b amp are similar, so i think the bc550/bc560 are fine. i ordered some irf610/rf9610 and change them tomorrow. also i think the pot could be 10k. the resistors are placed like printed on the pcb, i checked them 3 times now, i don't see a mistake in soldering/placing the parts.
ps the two c3 330pf are missing in the picture, had some trouble once with the one i have and desolderd them today / ordere some new one (don't think they have a great effect on the offset ;-) )
