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ForeWatt Tube SRPP Preamplifier - Support / Update Thread
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Author:  gofar99 [ 01 Jul 2020, 21:08 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

Hi, IMO the best way to balance the different gains from power amps is as mentioned earlier. Putting variable resistors in the inputs of the power amps or if not practical in a sort of junction box going to them. I would add an isolation resistor between the Forewatt output and the junction box though to minimize interaction between the power amp levels. Most likely I would use 100K isolation resistors and 100K pots. Since most power amps are designed for 1-2 volt input signals the Forewatt can easily drive them (it can deliver around 20V RMS clean). If you have solid state power amps you may have to change the resistor values. If the power amps have a fixed input resistor you might be able to just replace it with the pot (still use isolation resistors though). I would not go much below 10K though to prevent loading on the Forewatt. It will work as low as 2.5K loading, but It will not deliver the 20V output level. BTW I have used a Forewatt with a quad of tube power amps using this method and it works fine.

Good listening
Bruce

Author:  Rytikar [ 06 Jul 2020, 14:17 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

Hi,
thanks for the tips so far. I will place the gain control in the power amp side.
I may give a bit more detail. The amp I use now is a SE KT88 (Kegger,Blueglow) and it plays amazing together with the Forewatt. I just built myself new high-end speakers and I can hear the KT88 reaching it's limits when turning louder, especially with bass heavy music. That was expected, and I made the speakers bi-ampable and tried a 2*100 watt ss integrated (Thule IA100) and his volume control for the bass, but was not satisfied. Now I'm actually building a Gainclone to experiment.
I learned the gain of the LM3886 can be set adjusting the feedback resistor. Is this a way to go or will I waste the 'oomph' of the solid state amp needed for the bass this way?

Thanks

Author:  gofar99 [ 06 Jul 2020, 15:11 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

Hi, A gainclone is a decent amp. I used one for a while a number of years ago . The biggest issues with them is that they can oscillate. The gain is indeed adjustable. For sub woofer use I would probably alter the NFB to cut the high frequency gain down. Mine measured pretty flat to about 400KHZ. I would bring that down to about 5-8KHZ to cut noise and and enhance the stability. Even though I really like tube gear, I see that SS stuff is quite suitable for sub woofers. I am fortunate that mine are about 96 db/watt so I can easily get by with 20 watts on each.

Good listening
Bruce

Author:  Rytikar [ 07 Jul 2020, 05:42 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

Thanks,
I will see how the GainClone performs in the scenario. As said, it is an experiment, I had the cabinet and the transformer available, so no big cost involved.

On to another issue, while I'm here. :-)
Using my trusty phono preamp with the Forewatt I noticed a squealing, bubbling noise from one channel. Quite noticable and the sound is distorted. It never did that when connected to other equipment. Other sources work just fine with the Forewatt.

The design is from 1990, Schäfer and Rompf, opamp based, quite a quality build. I looked into the schematic and saw that the output comes straight from the opamp pin 6, nothing inbetween. I think something is oscillating, but have not measured while connected with the scope yet.
While measured 'solo' with a riaa adapter, the phono preamp shows no problems, so maybe a matching problem with the Forewatt?

Looking at other phono preamp designs (Bruce's ss and tube) there is a 4,7uF film cap at the output. I think as a high pass filter.

I plan adding those to my phono preamp. Anything else that could cause such effects?

Thanks

Author:  ILoveHiFi [ 07 Jul 2020, 07:41 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

Oscillation depensds on the schematic design, nonrmally a well designed shcematic will never oscillate over a pure resistive or speaker load.
Most chip amps if you wire it to datasheet recomendations and you purchase from repersentable geniune chip retailers, it will never oscillate and will perform well.

Not the best sound compared to expensive amps but it does it job the chip amps。

Well design schematic where it dosen' oscillate over a pure capacitive load of any value will normally sacarafice the sound quality, while majority of amps will oscillate when an pure capacitive load of 0.01to10uF is present, but speakers never presents a load like this always inductive.

To measure if something is oscilalting, first you need to set the scope on 1X attenuation, more than 1X attenuation the resistor divier and the capacitors installed will filter out the high frequency oscillation so you are not testing anytthing but wasting your time.
1X attenuation means direct input to scope.

If you increace the gain of any amp to make it louder, it will become more stable because of less negative feedback, amps and op amps with very small closed loop gain are more likeley to oscillate.

Author:  Rytikar [ 07 Jul 2020, 10:05 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

Thanks @ILoveHifi,
I will play with the gain, most probsbly keeping it high, and/or add a volume pot at the input of the GainClone.

I think I was not clear enough when switching topic. :-) The squealing noise, suspecting oscillation, comes from a phono preamp connected to the input of the Forewatt.

Author:  Rytikar [ 21 Jul 2020, 07:33 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

Problem solved,

after tinkering with the phono preamp it was the interconnect cable in the end.
Brand new Oehlbach, oh well...

Thanks

Author:  Rytikar [ 24 Jul 2020, 04:48 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

@gofar99, what exactly do you mean with '100k isolation resistor' and where to place them?
I played with the gain adjustment on the LM3886 and that worked, but drove the gain to under 10, what lead to distortion.
So I readjusted the gain to previous level and added a pot to ground to the input just as a normal volume control and that works fine as well. I'm just in doubt where to place the isolation resistors. And should the be on the output to the solid state amp only or on both?

Thanks

Author:  gofar99 [ 24 Jul 2020, 08:40 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

Hi, I had to look back to see what I had in mind....The 100K would be in series with the input of your pot and the output of the Forewatt. Essentially it would form a voltage divider at cut the signal to 1/2 as much going into the 3886.

Good listening
Bruce

Author:  Rytikar [ 26 Jul 2020, 15:21 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

Hmm, that did not work as expected, I'm getting not enough signal to adjust the pot to the correct gain.

I used a 100k resistor and a 100k pot. The resistor was conneted in series to the input of the amp, coming from the forweatt. The other side of the resistor was conencted to the input side of the pot. The other side to ground and the wiper to the Gainclone input.

When I give the Forewatt a 1khz, 500mvPP signal and adjust the volume on the forwatt so the KT88 outputs 1Vrms, I'm only getting about 300mVrms out of the Gainclone with the pot maxed out, even though the gain of the Gaincclone is at least twice that of the KT88.

Something wrong or misunderstood?

BTW, testing with the pot only removing the resistor there is quite some influence visible between the two outputs when adjusting the pot.

Thanks

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