DIY EAR 834p phono stage - a short evaluation/ test with those chinese PCBs

[Pillo69]

Hi.
I intend to replace the Phono PCB of the Dynaco Pass with an EAR834P (for its location I must make the PCB with special measures).

I asked in the guide thread about the final scheme "Thorsten mod."
http://www.lencoheaven.net/forum/index.php?topic=26658.45
The power supply is not an issue I'm worried about right now.
Do you think the following scheme is correct?



For the output resistance (220k), I must substitute for values according to potentiometer (resistance of 68k and potentiometer of 50k) ?.

The line input in the PASS preamplifier has a potentiometer of 250k, although in some places it is recommended to replace it with a value of 100k, this is not correct with the use of phono stage, modifying sound response.

What would be the combination to use a 100k volume potentiometer?

I appreciate the interest of the community.

[Jay]

Just a thought but if silicon extends below windings to base or lower edge of can then only a fairly thin ish layer of sorbothane would be required. Isolating or rubber mounting the centre bolt might also help reduce any mechanical vibrations.
Mu metal is horrible stuff to cut etc & is exspensive. That's why I thought of the Copper tape as it's easy to apply. Was asking if using copper would have any benefits.

[Jessica_K]

Hi Jose,
The actual output impedance of the amp is quite low ( few 100 ohms depending on the V3 valve used and it's cathode resistor). The 220k is there mainly to bleed the 1uf and link it to gnd. The result is the the impedance of any pot or amp input from 50k to inf would have very limited effect on the sound though I would recommend 100k plus as I have noticed a very slight effect on the frequency response as the impedance is reduced.

Robert

[Pillo69]

Thank you Robert.

It then gives a better result to replace the 220K resistor with a 68k resistor, and replace the 250k preamplifier input potentiometer with a 100K?

[Jessica_K]

No Jose, leave the 220k, as it is parallel with the 100k pot giving a load impedance to the 834 of 68k. If you change the 220k for a 68k then the load would be 40k and that's a bit too low as a ratio of output impedance to load

Robert

[Pillo69]

No Jose, leave the 220k, as it is parallel with the 100k pot giving a load impedance to the 834 of 68k. If you change the 220k for a 68k then the load would be 40k and that's a bit too low as a ratio of output impedance to load

Robert

Thank you Robert.
I'm going to design the PCB, and observe if all the components come in.
Dimensions must be 140x85mm.

[Jessica_K]

Curios, what CAD package you using for the PCB design? Are you going for a 4 layer board or 2.

You can of course mount the resistors vertically that will save you a lot of space as their size is not for power handling but for temp stability so don't need a lot of air around them.

Robert

[spaceistheplace]

Just a thought but if silicon extends below windings to base or lower edge of can then only a fairly thin ish layer of sorbothane would be required. Isolating or rubber mounting the centre bolt might also help reduce any mechanical vibrations.
Mu metal is horrible stuff to cut etc & is exspensive. That's why I thought of the Copper tape as it's easy to apply. Was asking if using copper would have any benefits.

Yes I purchased the sorbothane before having the silicone idea. I didn't want to mess with the epoxy, personally, and didn't think the returns would justify the efforts. Silicone was simply easier and is more pliable so I can get a uniform flat finish onto which I can place the sorbothane and couple it to the chassis baseplate.

I did not intend to use mu-metal or copper shielding. I figured the steel cover to be sufficient and like you said it'd be difficult to work with and perhaps come out looking rather amateur.

I might cover the divider between the amplifier section and PSU with copper foil tape however. That part has yet to be realized though. Taking this one slowly and with focus as I'd like to keep this one as my "reference" build.

[Pillo69]

Curios, what CAD package you using for the PCB design? Are you going for a 4 layer board or 2.

I have the Eagle Cad, although I do not use it yet.

I belong to "the old school", I make my PCBs manually, an AutoCad drawing program and chemicals (hydrogen peroxide and nitric acid).

[Pillo69]

I just saw that the input load resistance is 51K. The standard resistance for MM cartridge is 47K.
What affects this change ?.

[spaceistheplace]

It was the original input resistor of the commercial 834. You can use 47k as well if you'd like. The real input resistance should be adjusted for your Phono cartridge, whether it be mm or mc.

The change is immaterial in a practical sense.

47k isn't necessarily "ideal" for all MM carts. A lot of ink has been spilled on this in various forums.

For example, the frequency response is best with the Shure M97 at 68k and 100pf input capacitance (usually supplied by cable capacitance). Basically smooth with a little under 2db at 20Khz, and the -3db point is at 23Khz.

What cartridge do you intend to use?

You can load downward from 51k using the extra rca jack solution seen in the build guide.

The build guide also has links with further reading on ideal loading configurations for various cartridges.

[spaceistheplace]

A couple of progress shots finally getting going here with the 3U Pessante suggested in the build guide.

Now that I have everything drilled I'll start with some photos soon to give a step by step assembly guide....

As you can see I'm fairly clumsy and even I managed fairly well- nothing looks *too* lopsided lol.

I'll do my best to polish and take some higher quality / better lighting photos for the guide.

[spaceistheplace]

Question: is there any benefit or difference in wiring up the mains from the transformer in these two ways?:

1. Hot primary from transformer to fuse, from fuse to switch, from switch to IEC

Or

2. Hot primary from transformer to switch to fuse and from fuse to IEC?

Seems it would be less messy with wiring to use the first method but I generally see the second method used. Why? Any reason?

[Jessica_K]

I use the third option
TX primary to switch to a fused IEC
Kinda reduced option 2

I don't see any real difference I guess having the fuse first from the mains adds a small additional protection within the box

Robert

[Loggie]

John your build taking shape very nicely.

Do you think the copper tape will give extra shielding.....by that I mean more shielding than the enclosure itself?

Looking at the Pessante it has more room than my enclosure so I might be switching to your advise.
When your build and James his build have been completed I'll make my final version by using the best tips and tricks you guys incorperated

Looking forward to your next update John !

-Arno