DIY Audio Projects Forum

Check voltages across fets.

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Projects: “Mantra” - tube MM phono preamp | "Carbon” - MM phono preamp | 180W RAW Class D amp | "Illusion" - 6SN7 SRPP preamp in a plastic jiffy box |

Thanks for the prompt advice - yes only 12.45V across one of the FETs on the channel with less gain. So replace FET presumably? (or test it properly once removed from board?)

Hi chaps, I'm new here. Been dabling in electronics as a hobby on and off for years, and have now got one of these kits on order. I do however have a couple of questions!

Fistly, the resistive loads issue:


I have a Dual 505-4 TT with a Ortofon ULM 68E cartridge, but cannot find any specs on the resistive load. It's an MM cartridge so I'm thinking 47k, but what if this is not correct? I'm therefore inclined to swap out the 100k resistor with a 100k Pot, or perhaps a 25k fixed and a 100k pot, to allow for a bit of flexibility - any thoughts or comments?

Second question: I'm looking to power the amp with a switchable mains/transformer, or on board battery power source. In order to limit the number of banks of AA (Ni-Mh) cells I'll need, any idea what the minimum operating voltage of this kit is? Would something like 24v be OK? Can I go lower?

Many thanks in advance for your help

Peter

Billywink: I would definitely use a 47k resistor to load the cartridge. Don't use a pot here they are too noisy. Reading my remarks above the loading for a MC cart. Is 100ohms not 100k. My Ortofon Rondo Blue is loaded to 50ohms but sounds just as good at 100ohm loading.

So use 47k. You really need to stay with 30V for the PS because there is a restive network which provides the right voltage to the tube filaments. Varying the PS will vary this voltage and stop the tube working properly.

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Projects: “Mantra” - tube MM phono preamp | "Carbon” - MM phono preamp | 180W RAW Class D amp | "Illusion" - 6SN7 SRPP preamp in a plastic jiffy box |

Excellent! many thanks for that.
Peter

Ok, just built this pre-amp and fired it up for e first time tonight. I am getting terrible humming/buzzing. I don't know where to start to solve the issue.

This can be a very "hard to pinpoint" type of problem, so lets start with the basics:

1) SHIELDING: The preamp requires good shielding to prevent it picking up stray AC fields. It should be isolated as much as possible from its power supply (which is why the power supply section of the PCB can be separated from the rest).

2) GROUNDING: DO NOT make common direct connection of the preamp to earth ground (mains ground). It should be separated via 150R and 100nF X2 capacitor (in parallel) between mains and preamp ground. If you've used a metal enclosure (personally recommended), connect the mains ground to the case and then place the resistor/capacitor pair between the preamp ground and case.

Hum noise can be induced on to the in/out signal cables if the shielding of those cables is bad - or - the is poor ground connection at the jacks. HOWEVER, to avoid ground loop situation, be sure that the ground from the PCB DOES NOT directly connect to the shielding of the interconnect cabling. The best approach being to use either small shielded wiring from the preamp PCB to the in/out jacks or tightly twisted pair wiring, but leaving the shield(ground) unconnected at the jacks (only connected to ground at the PCB end) or vice-versa.

Good pictures of the project may help as perhaps we can see something in the wiring.

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The key to a successful build is to keep the smoke IN the circuit.
-Les

We cannot solve our problems with the same thinking we used when we created them. - Albert Einstien
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LM380 Bridged Guitar Amp, Oatley K301 Phono Pre-amp, Oatley K272 Headphone Amp, Tube proto-board

Well feeling pretty dumb here, but where should I use sheilded cable. Where do I get sheilded cable. I just used some 22ga wire from radio shack. I will take some pictures and try to get them uploaded as soon as I get time.

Also, where do you drop grounds from the PCB to the chassis?

How did you connect your source to the preamp and then your preamp to your amp?



On the preamp PCB, you'll see connections labeled "GND". Use any one, and only ONE, to make your chassis connection.

_________________
The key to a successful build is to keep the smoke IN the circuit.
-Les

We cannot solve our problems with the same thinking we used when we created them. - Albert Einstien
_________________________________
LM380 Bridged Guitar Amp, Oatley K301 Phono Pre-amp, Oatley K272 Headphone Amp, Tube proto-board