Foreplay II excessive gain- amp pairing

choff · 3993

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Offline choff

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on: November 29, 2022, 02:35:01 PM
I have a Foreplay II with all the upgrades. Recently purchased a Primaluna EVO 400 power amp and I am getting a lot of noise with no music playing on my Klipsch Forte III’s.  Also, can’t go over about 10 o’clock on the stepped volume pots without it being too loud.  Added a 6db in-line attenuator at the RCA amp input.  Seems to have helped, but I still get some hum buzz from pre-amp that is noticeable on quiet passages and annoying when no music is playing.

Should I step up the attenuation by buying 10db or higher Rothwell or HB labs in-line attenuators?  Or is there another option / problem to seek out and fix?

Thanks!  C.



Offline Paul Birkeland

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Reply #1 on: November 29, 2022, 02:49:25 PM
Man, you have a preamp with a lot of gain, an amp with 27db!! of gain, and speakers that will go to ear bleeding levels with a watt or two. 

If we believe Klipsch's data and work backwards, 2W gets you to 102dB, 4W gets you to 105dB, which is exceptionally loud for home listening.

4W into 8 ohms is 5.6V.  To get 5.6V out of your amp, you need 0.250V of signal at the input jacks.  A really, really weak source will put out 0.3V of signal, so a preamp with any gain is too much gain. 

At the bare minimum, I would go with a pair of -12dB attenuators to help get things under control.  Different speakers or a very low gain power amp (if you want a push-pull amp, the Quicksilver Horn Monoblocks are made to work in this situation).

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline choff

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Reply #2 on: November 29, 2022, 02:58:34 PM
Thanks Paul.  Maybe I made a mistake buying this amp. I was getting really loud pops when switching the volume before adding the 6db attenuator. Hope I didn’t damage the speakers?   I had a Dynaco 70 that I used with the Foreplay for almost 20 years and loved.  Rothwell makes 15db and 20db in-line. Are either of those better options?  Or is a doomed pre-amp, amp and speaker combo?



Offline Paul Joppa

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Reply #3 on: November 29, 2022, 03:09:44 PM
I'm currently running Eros-->Foreplay III-->Paramount-->101dB speakers, and I use -12dB attenuators at the Foreplay input and again at the Paramount input - yes, a total of 24dB attenuation. It works well for me.

Paul Joppa


Offline choff

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Reply #4 on: November 29, 2022, 03:23:23 PM
Good to hear. I have a Seduction and cd player running to the Foreplay II. Would it help to attenuate both of those inputs to FP equally?  Maybe I could pick up another pair of the 6db for this purpose and then a 12 or 15db pair for the amp input?  Is it a problem to use those higher 15-20db attenuators?  Seems like most stick to 10-12db as you have for some reason?  Does they start to roll-off the highs? Thanks.



Offline Paul Joppa

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Reply #5 on: November 29, 2022, 08:17:29 PM
At the time I bought them, I think 12dB was the biggest attenuation offered. There are no theoretical downsides to greater attenuation as long as the impedances work in the system. On the plus side, greater attenuation improves the input:output impedance ratio.

The problem with impedances is still real though. I have been able to find very few measurements and zero manufacturer's specifications of the input and output impedances of these devices. Tube preamps prefer to drive a much higher impedance than many solid state devices offer, so that's important. The risks are (1) loss of bass, unless the output capacitor of the driving device is increased, (2) increased distortion, and (3) current clipping if there is a cathode follower. High frequency loss is uncommon in practice.

I have three data points:

1) I bought some inexpensive attenuators sold for use in car stereos, which turned out to have very low input impedance, around 1K-2K ohms IIRC (it was a long time ago!) - far too low for most tube preamps to drive comfortably.

2) At the same time I bought some Harrison Labs 12dB - the ones I use now. They have a 10K input impedance, which is uncomfortably low but will work with at least many preamps, including Foreplay, if you don't push them too hard.

3) The Rothwell 10dB attenuator has a 30K input impedance, as measured by someone on the Audiogon forum. Almost any tube preamp can drive that happily. I don't know the impedance of their 20dB attenuator, but it's likely in the 30K-100K range. They cost more, and come from England, but the dollar:pound rate is favorable at the moment  :^)  Maybe I should snag a few for myself ...

Incidentally, Schiit's $50 passive preamp is 10K and has a volume control for adjustable attenuation. You can use this to determine the ideal attenuation, and then if you wish modify the inputs with fixed resistors to match the setting and optilize the impedance as well:

https://www.schiit.com/products/sys

On the question of preamp inputs, CD players often have 6 to 10dB more output than typical MM phono preamps (including Seduction/Reduction) with typical 5mV cartridges, so you may find you just need one on the CD input to make the volume settings more similar.

Paul Joppa


Offline choff

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Reply #6 on: November 30, 2022, 04:34:16 AM
Thank you Paul. This some really helpful information. Sounds like I should opt for the Rothwells at amp input.  Just need to determine the best value to use. The passive pre-amp seems like a good option. How exactly would this tell me the proper in-Lin attenuator to use?



Offline Paul Joppa

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Reply #7 on: November 30, 2022, 06:30:36 AM
Once the desired attenuation is found, measure the input and output DC resistances. From that, you can calculate the attenuation - 20*log(Rout/Rin).

Paul Joppa


Offline choff

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Reply #8 on: November 30, 2022, 10:03:45 AM
Ok. I think I follow.  One thing that occurred to me is I am less concerned about having a broad volume control as I am with the amount of noise coming from the pre-amp causing the background noise.  I tried a number of experiments removing the input cables to the pre-amp to see if they were the cause. The noise is unchanged whether the cd player or phono stage are connected.  Nor does it matter how much I raise or lower the volume when no music is playing. It’s a constant low level hum buzz that is most bothersome. Seems to me this can only be corrected at the input connection to the amp with a strong enough attenuator.  Is this correct?



Offline Paul Birkeland

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Reply #9 on: November 30, 2022, 11:14:54 AM
Do you have the ground jumper modification installed on the Foreplay II?

Is the noise a 60Hz hum or a 120Hz buzz (try listening to a test tone at each frequency to help determine that).

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline choff

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Reply #10 on: November 30, 2022, 11:34:14 AM
Yes, I did the jumper mod not long after building 20 years ago.  Not sure if I can do that test you are referring to. Sounded near dead silent with the Dynaco 70 and the 6db attenuator prior to this new amp.



Offline Paul Joppa

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Reply #11 on: November 30, 2022, 12:35:58 PM
The ST-70 has a gain of about 13dB on the 8 ohm tap (spec is 1.3v input for 35 watts out) That's 14dB less than the Primaluna EVO 400, so a 14dB attenuator would make it work like an ST-70. A 19dB attenuator would make it match the ST-70 with 6dB attenuator.

Paul Joppa


Offline choff

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Reply #12 on: November 30, 2022, 01:05:35 PM
Wow.  Thank goodness for this forum. I would never have the ability to do the calculations you provided.  So it sounds like I am best going for the 20db attenuators? 



Offline choff

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Reply #13 on: November 30, 2022, 02:49:21 PM
Curious.  How do things change if I were to use the 4ohm tap?



Offline choff

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Reply #14 on: November 30, 2022, 02:59:00 PM
Thanks to both Paul’s for the invaluable help with this. I am tempted to chance things and try the 15db attenuators as there were times with the Dynaco where I would hit the limit as far as how high I wanted to turn it up but there was no more room in the Foreplay volume pots to go.  Usually with records or old low mastered cds.  Just hope that noise floor is canceled out or close with the 15db as it is pretty distracting.  Hard choice. Worst case is I waste another $60 I guess….