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PostPosted: 28 Dec 2012, 01:54 
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omni88 wrote:
I just built this kit last night and wonder if anyone has had an issue with distinctly noticeable channel imbalance (definitely the preamp). I was fortunate in that I had two very closely matched pairs in the five tubes supplied so CH1 got two tubes with 134 & 88.2 uA respectively and CH2 got 133 & 92.4 uA. I'm certain I did not misplace them on the board having marked them up beforehand (the obvious thing to suspect in the first instance). I don't know if it is valid to perform the matching test again on a populated circuit board, but for what it's worth even though the values were different (much closer) repeating the test on the mounted tubes yielded CH1: 108.5 & 107 uA, and CH2: 108.7 & 107.8 uA.

It's a pretty straightforward build and I have double checked all the other components are placed correctly. Does anyone have any ideas please? should I test the FETs? Only change I made was to use a 12VAC wall wart instead of the supplied PCB transformer since I had one and moving the transformer further away seemed like a good thing. Hum isn't an issue.

Check voltages across fets.

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Projects: "Lagoon" - tube preamp with cathode follower | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
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PostPosted: 28 Dec 2012, 06:07 
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mwhouston wrote:
Check voltages across fets.


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?)


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PostPosted: 04 Jun 2013, 16:01 
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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:
mwhouston wrote:
beaubbe wrote:
I was wondering if anyone knows what is the stage resistive load for this preamp. I'm somewhat of a newby when it comes to this, but from what a read and understood from the schematics of this amp, the resistive load is specified by the R1 (or R101) resistor. So that would make a 100k stage resitive load for this preamp. Is that right?

If you are talking about the load on the cartridge it is decided by R1 (R101). On the original schematic that is 100K which I would say is too high for most MM cartridges. My suggestion would be to make R1 (R101) 47K. Look to your cartridge specs. for the suggested loading. It can vary quite a bit.

If you have a MC cartridge I would leave R1 (R101) at 100K. The MC preamp will provide loading to the cartridge. If you don't have a MC preamp take a look on this forum for my Le Pacific MC preamps or Hiraga MC preamp.


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 :beerchug:

Peter


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PostPosted: 04 Jun 2013, 17:27 
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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: "Lagoon" - tube preamp with cathode follower | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
Website: retro-thermionic


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PostPosted: 06 Jun 2013, 03:54 
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Excellent! many thanks for that.
Peter


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PostPosted: 03 Aug 2013, 23:03 
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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.


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PostPosted: 04 Aug 2013, 15:33 
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NuttyLemur wrote:
Ok, just built this pre-amp and fired it up for the 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.

:beerchug:

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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


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PostPosted: 04 Aug 2013, 18:53 
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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.


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PostPosted: 05 Aug 2013, 18:01 
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Also, where do you drop grounds from the PCB to the chassis?


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PostPosted: 05 Aug 2013, 18:13 
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NuttyLemur wrote:
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.

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

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


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


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