[Solved] Hum noise on pre amp vintage
Hi DIYers!
I upgraded an 80's receiver, which has the obsolete SANKEN SI1340H in the outputs.
I put a pair of APEX AX14 in its place and the sound is fantastic! However, the "hum" is very evident when there is no sound. There is no difference in changing the volume control, only when the bass control increases. It uses the HITACHI HA1457 as pre amp.
At first, it looks like a ground loop. So I refined and improved the star grounding.
I replaced some pre-amp and power supply electrolytes, but "hum" remains. 😡
I imagine that this HA1457 pre amp has a high output (gain) level. I would like to know how to lower the HA1457 gain or other more appropriate solution.
Thanks for all the help and sorry for my bad English! 😀
Edit:
[Solved]
I decrease the APEX AX14 gain: I changed the R8 resistor value from 560R to 1K. All the noise disappeared. 😀😎
Hi DIYers!
I upgraded an 80's receiver, which has the obsolete SANKEN SI1340H in the outputs.
I put a pair of APEX AX14 in its place and the sound is fantastic! However, the "hum" is very evident when there is no sound. There is no difference in changing the volume control, only when the bass control increases. It uses the HITACHI HA1457 as pre amp.
At first, it looks like a ground loop. So I refined and improved the star grounding.
I replaced some pre-amp and power supply electrolytes, but "hum" remains. 😡
I imagine that this HA1457 pre amp has a high output (gain) level. I would like to know how to lower the HA1457 gain or other more appropriate solution.
Thanks for all the help and sorry for my bad English! 😀
Edit:
[Solved]
I decrease the APEX AX14 gain: I changed the R8 resistor value from 560R to 1K. All the noise disappeared. 😀😎
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Set the AX14's gain to exact the same value as the originals' (~26.5) and report again, please!
Best regards!
Best regards!
Can you say if the hum is at mains fundamental frequency (50 / 60 Hz - I don't know where you are ) or double that ( 100 / 120 Hz ) ?
if it is coming from the HA1457, it would have been there before.
reduce the gain to 15 and try again. I have a HA1457 in the phono stage and it is a good chip.
remove the input capacitor ( C601 ) to the AX14 and check for hum.
You have to put the whole pcb module in as the main board only has the NFB loop of 1 +(56k / 2.2k) for gain of 26.5.
that's a nice reciever similiar to the pioneer sx-3700 I have
currently I am installing a Litchstark-x amplifier into an sx-780. this amp is in another thread here
this project requires a daughter card mounted on top of the main pcb.
the HA1457 requires an adapter to convert from it's 9 pin SIP to 8 pin DIP. pin 2 is missing on this IC.
reduce the gain to 15 and try again. I have a HA1457 in the phono stage and it is a good chip.
remove the input capacitor ( C601 ) to the AX14 and check for hum.
You have to put the whole pcb module in as the main board only has the NFB loop of 1 +(56k / 2.2k) for gain of 26.5.
that's a nice reciever similiar to the pioneer sx-3700 I have
currently I am installing a Litchstark-x amplifier into an sx-780. this amp is in another thread here
this project requires a daughter card mounted on top of the main pcb.
the HA1457 requires an adapter to convert from it's 9 pin SIP to 8 pin DIP. pin 2 is missing on this IC.
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I disconnected the APEX AX14 preamp stage and the "hum" continues. I connected the preamp output directly to the APEX AX14's input, removing R601 and C601 and the "hum" continues. Sorry for my poor knowledge, but how to reduce the gain? On HA1457 or APEX AX14? Thank you!
The values of R2 and R8 are correct, as you mentioned. I even reduced R8 from 560R to 330R to decrease the gain, but no changes.
330R will increase gain 1k will reduce gain.
The (s)ground pin on the AX14 to the center of the large ripple capacitors.
speaker ground and power transformer center tap is here. The hum should be eliminated
The (s)ground pin on the AX14 to the center of the large ripple capacitors.
speaker ground and power transformer center tap is here. The hum should be eliminated
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These values are in what places?
I made this adapter to another device of the same model. I used OPA604. The sound is very good! I have several devices of this same model! Hehehe
A big mistake! So I'm going to try 820R on APEX AX14's R8 and see the difference.
I changed the value from R511 to 100K. Another great detail: A bridge diode had blown up! I changed all of them, having exchanged and increased the electrolytic value from 8200uF to 10000uF. I soldered the central tap of the transformer directly into the center of the star ground. Originally, the wires are wound on terminals. I think they have gained some resistance over time ... I think I will try to reconnect the RC filter C602 (330pF) and R602 (56K) and I will note the difference. Some "hum" in the background still persists, but now it is above 95% solved!
And we will continue to revive the vintages! 😀 Thank you!
the 56k, 2.2k and 330pf cap form the NFB loop for the old chip. connecting that to the AX14 will duplicate the function of the similar loop in the AX14. It should be removed.
Usually an amp won't run long with a shorted power supply diode. This does explain the main source of noise.
The AX14 should be silent with no input and the signal ground tied to the star ground point.
Usually an amp won't run long with a shorted power supply diode. This does explain the main source of noise.
The AX14 should be silent with no input and the signal ground tied to the star ground point.
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Ok, I had bypassed these components.
I notice that it picks up a lot of noise from anywhere.
I'll follow your suggestion and increase the value of R8.
The phono stage also uses the HA1457, but it looks like it's okay.
I'll do that tomorrow and post the result here.
Thankful!
Hi,
Today I changed the R8 value of APEX AX14 from 560R to 1K and heard practically no noise! Maybe I'll try 1K2. I will seek perfection!
Thank you all!!!
Today I changed the R8 value of APEX AX14 from 560R to 1K and heard practically no noise! Maybe I'll try 1K2. I will seek perfection!
Thank you all!!!
Thiago, are you from Brazil?,I need some help, trying to do exact same thing you did, put a new amplifier, but i'd like to preserve the vu meter, have you tried that? Thanks.
André Gozetto
André Gozetto
Hi. I repaired one SI1340H with the same symptoms. The problem was caused by a 2SA812. In the picture is pointed the replaced transistor. I think that in case of failure, all the smd transistors are suspiscious. They are 2SA812 and 2SC1623. In the transitor datasheet, number in marking code is asociated to hfe data.
Is paradogic, but for to know how to open the SI1340H I broke the other good IC. For to open the IC, aply hot for to burn the glue, then you open the IC like a oyster. Then, applyn hot in the pins for to remove, because the pins are bonded to the plastic cast. You have to remove the pins from the plastic cast later.
Is paradogic, but for to know how to open the SI1340H I broke the other good IC. For to open the IC, aply hot for to burn the glue, then you open the IC like a oyster. Then, applyn hot in the pins for to remove, because the pins are bonded to the plastic cast. You have to remove the pins from the plastic cast later.
