Foreplay III New Build + adjusting listening levels + ST-70 troubleshooting

[syncro]

Just completed the build and beginning the resistance checks.  Power transformer terminals 1 and 2 read "0".  This is per spec, but the others that should read "0" do not.  Terminals 6, 7, 9 and 10 all are reading with variable values.

I'm about half way through the other resistance checks and they are looking good.  What should I do to get help - finalize my resistance checks, post them all...?

Thanks in anticipation,
David

[Paul Birkeland]

The measurements of the power transformer terminals vary a lot between meters.  If everything else looks OK, go ahead and move onto the voltage checks. 

[syncro]

Thank you very much for the direction - exactly what I was hoping to hear.  I am using an Amprobe 35 XP-A DMM.  I am new to using a DMM. 

I like the continuity of a needle because that's what I'm used to, so I was about to put my trusty 1970's Micronta (Radio Shack) analog multi-tester to work for verification.  I will post any further discrepancies, or listening notes as the case may be, here. 

[syncro]

All resistance and voltage tests checked out okay, and the sonics are wonderful!  Thanks again for the support.   After I listen for a while and get multiple inputs connected I'll be looking for advice on how to best adjust output for use with my Dynaco ST-70. 

[Paul Joppa]

...  After I listen for a while and get multiple inputs connected I'll be looking for advice on how to best adjust output for use with my Dynaco ST-70. 

Congrats on a successful build!

While you are listening and getting other things attached, read my paper of signals and noise (linked from the community page). The ST-70 has more gain than you need, unless your speaker are extremely inefficient - you'll probably want to attenuate at the ST-70 input.

[Grainger49]

The first bit of advice with a ST-70 is to remove the two 10 ohm resistors that are between the input common and the signal common.  Replace them with a short.  These very often cause hum.

The output of a FP III is pretty compatible with a ST-70.  You may find you want additional padding so you can use more of the volume control.  Otherwise they are a good match.

[syncro]

Thanks, and thanks.  Yes, I'm happy to have a successful build, and I do want to adjust something so I can get the first position listening level to be very low.  Now it is about where I would think a 3rd or 4th position should be.

Mr. Joppa, I will read your paper again and hope it helps me decide which place to "pad" things.  I saw in the "Easy volume control" thread a suggestion for padding the FPIII with resistors between selector and the attenuators in place of the red wire.  So, it sounds like I could either do that, or something similar in the ST-70, correct? 

Mr. Morrison, does the removal of that pair of resistors in the ST-70 "attenuate its inputs"?  I need to go into it anyway to replace a the tube sockets on the PC board.  (The amp is stock but with some upgraded capacitors.)  I'll move this inquiry to the ST-70 discussion forum to find out their recommendations for lowering the gain in the Dynaco.  Unless you all think I should keep asking questions here.  I'll need some hand-holding, as I'm a lay-person in this world of audio electronics. (between input common and the ?"signal common"? I'll have to take a look and read up on how to drain capacitors...)

[Paul Joppa]

...
Mr. Joppa, I will read your paper again and hope it helps me decide which place to "pad" things. ...

Well, the paper is pretty dense. I tried my best but the subject is inherently complex and confusing, and I don't claim to be the most articulate writer either. If you have further trouble, just post - or you can ping me through the forum if you like. Meanwhile I'll try to outline the options. There are three:

1) Pad the preamp input. This is what's in the Foreplay manual. This is the right place if the source device has a large voltage output.

2) Pad the power amp input. This is the right place if the amp and/or speaker is too sensitive.

3) Pad the preamp output. This is not a good idea unless all the stars are aligned - it makes sense if the desired attenuation is much greater than 20dB, and the preamp input is not too high a voltage.

If you need help identifying the best option or combination of options, you must first locate the necessary specs. I'll put them here, in case you or other people need to refer to them through the search function:

* source output level. CD players are usually specified by their full scale (FS) voltage, while vinyl and tape are specified by their average level, e.g. vinyl is referred to 5cm/sec groove velocity. In the case of vinyl, the cartridge sensitivity and the phono preamp gain are both necessary specs.

* Amp power (35 watts per channel for a ST-70).

* Amp sensitivity. This is usually the volts input required to generate the rated output power, though sometimes it is the voltage needed to generate 1 watt. For example, a quick Google search told me that the stock ST-70 is specified to produce 35 watts with 1.3vRMS input.

* Speaker sensitivity, usually dB at 1 watt at 1 meter. Sometimes this is given as dB at 2.83 volts, which would be 1 watt if the speaker were 8 ohms exactly (very few are!)

Good luck, and call on your buds at the forum (here!) if you have questions.

[Grainger49]

David,

The resistors do not attenuate the input but was there because equipment "of the day" had grounding problems.  Today with a FP III, and I know this from personal experience, it causes hum. 

Go here:   http://www.curcioaudio.com/st7_mnl.pdf

You can save this file onto your computer.  The schematic is 8 pages down and it shows 2 - 10 ohm resistors on the circuit board all the way to the left.  What is contained on the circuit board is inside the dashed lines.  All the way to the left and outside of the dashed lines are the stereo/mono switch and two other 10 ohm resistors (and the two 470k ohm input load resistors).  These two 470k ohm resistors are where PJ suggests you can pad your ST-70's input. 

If you PM me with your email address I will send you a picture of my ST-70 in this area.  I have removed the stereo/mono switch and some other unneeded wiring.  I think I can find where on the board the 2 - 10 ohm resistors are.  Or you can just measure across it.  The color code should be Brown Black Black.

[syncro]

Thanks for the further details.  So, unrelated to the volume attenuation issue, I should remove these two 10 ohm resistors (the ones ON the pc board of the stock ST-70) because they cause hum.  In doing so will I need to short the two terminals of the resistor, or just leave the terminals disconnected?  (Thankfully, I'm not hearing any hum, hiss or noise of any kind, BTW!)

Related to the attenuation issue, again on the ST-70, the two 470Kohm resistors at the inputs can be padded with an "L-pad" circuit in place of the existing 470Kohm resistors?  (I found some variable L-pads at Parts-*xpress - stereo, 15, 50 or 100w versions.  Not sure if this is what you all have in mind should that location be chosen for attenuation.)

At this time I am trying all my inputs and gathering the data suggested.  While the CD and Phono (w/ Hagerman Ripper as phono preamp) seem loud, the Ripper as DAC source is very, very low.  I'm going to check with J. Hagerman about that next.  Then I'll bring the summary back to this thread.

Thanks again,  David

[Grainger49]

Yes, the two 10 ohm resistors on the PC board are what I'm saying to remove.  Replace them with a piece of solid wire or bring the wire from the input RCA jack outer conductor to a common/ground connection on the circuit board.  An easy way to do this is at eyelet 10 for one channel, eyelet 8 for the other.  I'm not sure which channel is which from the schematic.  Like I said in modern equipment these resistors are not needed.

The L pads you found are probably like potentiometers, if I am thinking of the right thing.  That isn't what you need.  You could just unsolder the 470k resistors at the center conductor of the input RCA jacks.  Also remove the input wire from the center conductor.  It is easy to identify the input wires, they go to eyelet 7 and 17 on the circuit board.  Then put in a series resistor from the center conductor to the end of the 470k that you just removed.  This leaves two resistors in series going to ground.  Then the input is taken from the "junction" between these two resistors; the solder joint you just made.  The input wires should reach since this point is closer to the circuit board.  The value of the series resistor will be determined later.

[JC]

I would advise not getting overly-focused on those two 10 Ohm resistors, particularly if you don't have any hum issues.  In my experience, they just don't matter all that much.

You can always try jumpering them with test leads to see if you can tell the difference.  Taking them out involves de-soldering from an old phenolic PCB, which is always to be avoided if possible, in my opinion.  If you decide the hum level is lower with them jumpered, it is usually easy enough to make that condition more permanent by using a bit of wire attached at each end of the resistor on the "part side" of the circuit board.  Solder lightly, and you're done.

Every case is different, but I have tried a few different pre-amps with my ST-70, and found no difference with a scope whether those resistors were there or not.

[Grainger49]

JC,

That is good advice.  Paully and I found the problem using a pair of jumpers.  His FP III and ST-70 had plagued him with hum.  Here at my house his ST-70 and my FP 2 had no problems.  My ST-70 and FP 2 never had a problem.  But finally his rig here hummed.  And I tried two jumpers and it was gone.

I have worked on the old circuit boards for a long time so I have skill getting things off and back on.  The inexperienced shouldn't try it in their main system.

[JC]

Yeah, that's the thing about hum, isn't it?  Something that works perfectly in one situation hums the instant you try it in another situation. 

You will recall that safety ground and circuit ground on the FP2 were separate, but with the option to connect them.  In my particular situation at the time, I found that actually using a ten Ohm resistor between the two resulted in less hum, in that particular set-up.

Of course, that was before I installed a three-wire AC cable on the ST-70 it was connected to, so that may no longer be the best solution!

[syncro]

Okay, I'm back with my data.  Please help me determine where and how to attenuate.  I do have a little hum, but I have not done enough observation to provide any details on it yet.  The levels are high from all inputs and CD seems the highest, so this individual input might take it's own resistor after I get in the ballpark with the whole set up.

By the way, I'm really enjoying the sound qualities of this preamp.  Thanks for the kit and support.

Input 1:  Phono - LP12 w/ Basik Plus tonearm
Phono Cartridge:  Linn K5 (Moving Magnet)
Sensitivity:  4.5 mv Output at 5 cm/sec at 1 kHz

Phono Preamplifier:  Hagerman Ripper (SS, has volume pot - P2T3503)
Gain: 48dB   (Output Impedance: 320 ohms)

Input 2:  CD Player
Rotel RCD-975
Output Level:  2.0 Volts rms  (Output Impedance:  200 ohms)

Input 3:  PC > Foobar2000 > Hagerman Ripper (as DAC & no volume control)

Amplifier:  Dynaco ST-70 (stock)
Power output:  35 watts/channel continuous, 80 watts/channel peak
Sensitivity:  1.3 volts RMS input for 35 watts output

Speakers:  Linn Index I
Sensitivity is 86SPL, 1w/1m (impedance is 8 Ohms nominal, 6 Ohms minimum)

RE: digital input.  I have to correct my previous statement that the digital input was very low - I found a volume in my computer software that was lowered, and  Foobar2000 (my digital player software) has dB annotations on it's volume control.  Is that possibly accurate enough to determine the amount of attenuation I might try?

Thanks, David