QSXM3 Noise Floor Or Hum

[goldkenn]

Recently I brought my QSXM3 to my friend's place, he is currently using ASR Mini Basis and working fine with a Rega RB300 and ZYX R30 low output cartridge.

I don't know why when replace the ASR with QSXM3, tune the volume to the same level as my friend usually did, incredible hum comes output, we tried to change loading and gain and grounding but nothing work.

Other then hum, we found that the QSXM3 have much, more higher microphone effect, which it work like a very high gain amplifier.

Then we tried to change the cartridge to MM, a Denon DL-60, now can hear music but the QSXM3 sound higher sound level then ASR at the same MM setting, and at the same volume level, the QSXM3 did have an audible hum compare with ASR.

The intergrated amp is DIY, my friend claim that the gain of the amp is very typical, and the speaker is a ProAc Response One which also nothing high sensitive.

The gain of QSXM3 and ASR at MM should more or less the same at around 40dB, if there is nothing wrong with my QSXM3, I am thinking although the voltage gain is at the same level, is that the output buffer have higher current then ASR, giving the QSXM3 have higher driving ability, which in some case it work same as a high gain phono stage?

[peranders]

Do you have more than one point grounded in your turntable? Is it 50 Hz or 100 Hz hum?

[goldkenn]

Actually you can say there is no ground since Rega RB300 originally have no individual ground wire, but ground shorted with negative of RCA.

It sounds like a 100Hz hum.

I am confusing since other then audible hum, also heavy microphonic effect, but at MM setting, my QSXM3 is same 40dB gain as ASR, but this is just the voltage gain, that's why I raised a suspect about the buffer stage.

By the way, the QSXM3 work fine in my own system which preamp is Pass B1, and poweramp is  "My Ref" Rev C 3886, speaker is a Fostex 8" full range, my friend's ASR also work fine in my system.

[peranders]

The arm should be grounded at the pin 3 of the input terminal. The cartridge should be connected by two indiviual wires to each coil. Try to identify the ground loop.

If you short the inputs, does the hum go away?

[goldkenn]

I can't  modify the tonearm grounding since first of all that is not my tonearm, second is all REGA tonearm is originally factory config as no individual grounding wire.

I test again today, in my own system for get rid of tonearm grounding issue (Acousitc Signature Tango with SME Type IV and original tonearm cable), first using a Densen B150 integrated amp borrow from my another freind, to make sure the amplifier's gain is more or less reach the so call industrial standard, the result is here:
1) When set to MM, amp's volume tune to max, input shorted, no hum no noise
2) When set to MM, amp's volume tune to max, input open, no hum but noise like radio
3) When set to MC, amp's volume tune to max, input shorted, hum but no noise
4) When set to MC, amp's volume tune to max, input open, hum and noise like radio

When Connected to my turntable, all result the same as input open, and all hum and noise will be audible within 1M from speakers, when volume tune to half level.

Compare with ASR Mini Basis:
1) When set to MM, amp's volume tune to max, input shorted, no hum no noise
2) When set to MM, amp's volume tune to max, input open, no hum but baisc acceptable noise floor
3) When set to MC, amp's volume tune to max, input shorted, no hum no noise
4) When set to MC, amp's volume tune to max, input open, very little hum if almost stuck your ear on the speakers and baisc acceptable noise floor

When using my DIY B1 preamp and poweramp, QSXM3:
1) When set to MM, amp's volume tune to max, input shorted, no hum no noise
2) When set to MM, amp's volume tune to max, input open, no hum but baisc acceptable noise floor
3) When set to MC, amp's volume tune to max, input shorted, very little hum if almost stuck your ear on the speakers and no noise
4) When set to MC, amp's volume tune to max, input open, very little hum if almost stuck your ear on the speakers and baisc acceptable noise floor

With ASR:
no matter MM or MC, open or short, all no hum no noise

In addition, regardless of hum and noise issue:
ASR MM gain offically stated it is 40dB, QSXM3 MM gain also stated at 40dB, but no matter match with Densen or my own DIY system, QSXM3 have higher microphnic effect compare with ASR, test by knocking the turntable or tonearm.

[peranders]

B1 has the gain of 0dB. A phono amp has 80 or 60 dB gain in the 50 Hz area so hum pickup is obviously a bigger problem.

A BJT input has by nature more noise with a high source impedance (open input). A JFET has different characteristics.

How does your enclosure look like? Is it grounded somewhere?

[goldkenn]

Aluminium chassis, one chassis, 2 transformers with no ground wire, I added copper foil to shield the transformers, all connectors and screws isolated from chassis, single grounded from pin 3 to a grounding post and chassis by a AWG18 wire, chassis ground to AC earth via a loop breaker circuit as here:
http://sound.westhost.com/earth-f4.gif
I simply change the original 35A bridge into four IN4007 as phono no need go through large current.

I did also tried removed the braker circuit direct connect ground to AC earth and disconnected from AC earth, nothing change.

I am also thinking is that the aluminium chassis have no shielding effect? Since I do know alumunin only non-magenetic but not possible to shield RF/EMI, am I right?

[peranders]

All metals will shield magnetic fields in high frequencies but for 50 Hz you must have a metal which have good magnetic permeability such as iron, cobalt, nickel, chrome, my-metal (alloy).

What happens if you have the pcb without enclosure?

[goldkenn]

I will try without chassis, but since all internal wire are just fit, will need time for some re-wiring to work wihtout chassis

[goldkenn]

I think I don't have time to totally disassembling the QSXM3 since today I've got a new project, but if can't figure out the problem I may not attention to work..... therefore today I did a test as below;

I pull out a rusty iron chassis I used for building tibe power amp years ago from store room, since it is a big chassis I can fully put the QSXM3 with aluminim chassis together into the iron chassis, and grounded the iron chassis with the same ground point as the aluminium chassis.

I also loosen the mounting screw of the transformer, to aviod the hum was cause by any resoanace from the tranformer.

However, when MC, the hum still there, and noise floor still high.......