Fender FR-12 preamp mod : The Hissterectomy

Lysander said:
Looks spec-compatible with the TL084, and very low noise (3 nV/√Hz, vs ~18nV for the TL08x line). A bit expensive at $10 a pop, but i think it should work perfectly fine otherwise; this is not a precision design :)
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Thank you...been designing electronic controls for over 30 years and I’ve never liked monkeying with SMD. :)
It’s good that the source has been determined and that it is an easy mod. Thanks to both you and @WKSmith !
 
I've been operating with the belief that these things all have about the same amount of hiss and the varying reports of some being worse and some having basically no hiss are due to different tolerance levels between different people, differences in listening environment, differences in setup, etc.

I wonder if it's possible though, that the stock op amps might have, like other electronic components, imprecise tolerances? Those TL084s have a 18nV rating. But what if it's really a +/- (X) situation. If you get lucky your FR might have three on the low end of the spectrum. If you get unlucky you might have three on the high end. And most units would end up being "average." Possible?
 
Deadpool_25 said:
I wonder if it's possible though, that the stock op amps might have, like other electronic components, imprecise tolerances? Those TL084s have a 18nV rating.
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It's... possible. The venerable TL084 is manufactured by multiple companies all over the globe, and i imagine some would have better tolerances than others. My FR-12 had all Texas Instruments ICs.

If you're lucky and get a batch of lower noise ICs on your amp, it can definitely improve things. The cascaded design of the EQ means that small differences in noise floor will have a significant impact on the output; but lower or higher noise floors would still fall within tolerances for that particular opamp. This is stuff which should've been designed in.

Having said that... i still believe that inconsistent noise reports are just people's different tolerances to noise, or how controls (high cut, in particular) are dialed. This is a noisy preamp by design, and TL084 is not a great fit for it, IMHO.
 
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Lysander said:
I still believe that inconsistent noise reports are just people's different tolerances to noise, or how controls (high cut, in particular) are dialed.
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I think this is probably the most likely explanation too. So many people assume noon should be the default on all the knobs. It definitely should be for BMT, but the cut knob should default to zero. If folks are setting the cut knob to noon, then yeah it'll be way quieter than someone who has it set to zero.
 
Lysander said:
It's possible. The venerable TL084 is manufactured by multiple companies all over the globe, and i imagine some would have better tolerances than others. My FR-12 had all Texas Instruments ICs.

If you're lucky and get a batch of lower noise ICs on your amp, it can definitely improve things. The cascaded design of the EQ means that small differences in noise floor will have a significant impact on the output.

Having said that... i still believe that inconsistent noise reports are just people's different tolerances to noise, or how controls (high cut, in particular) are dialed. This is a noisy preamp by design, and TL084 is not a great fit for it, IMHO.
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I don’t know….mine measured fairly low….and multiple folks reported significantly different decibel readings….albeit subject to user error.
 
200man said:
I don’t know….mine measured fairly low….and multiple folks reported significantly different decibel readings….albeit subject to user error.
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My guess is that your unit may be one of the outliers that happened to get some particularly quiet chips. Totally a guess though.
 
It's tricky to measure the level of the hiss consistently. I spent some time with 3 different phones using software SPL meters and a dedicated hardware SPL meter. One app had 7-8db higher readings than another and the hardware unit didn't measure any noise floor difference at 3' unless I took off the windscreen. The hiss level was highly dependent on what angle the mic was pointed as well. The hardware unit seemed more directional than the phone too. Measuring on the grill is also a crapshoot since over the woofer I got around 40db and over the tweeter around 60db.

Unless someone had two different stock FR-x units that exhibited different amounts of hiss I don't think there's a reliable way to consensually validate the level.

-Aaron
 
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aflynt said:
It's tricky to measure the level of the hiss consistently. I spent some time with 3 different phones using software SPL meters and a dedicated hardware SPL meter. One app had 7-8db higher readings than another and the hardware unit didn't measure any noise floor difference at 3' unless I took off the windscreen. The hiss level was highly dependent on what angle the mic was pointed as well. The hardware unit seemed more directional than the phone too. Measuring on the grill is also a crapshoot since over the woofer I got around 40db and over the tweeter around 60db.

Unless someone had two different stock FR-x units that exhibited different amounts of hiss I don't think there's a reliable way to consensually validate the level.

-Aaron
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this is a good point. I went through some exchanges as I started with 1 - FR10...fast forward a few units and another exchange to end up with 2-FR12's. In total i've had 4 units. 2 of each model. I kept the 2nd pair of 12's. All four cabs had the exact same amount of hiss. I kept all my settings with the onboard EQ the same in each case. I keep my cut around 3 (9 o'clock).
 
Quick question at @WKSmith regarding the components used to update the preamp:
Would Wima "MKS 2" series caps be sufficient for the voltages inside the preamp? Those are rated at 63 VDC. Or should higher rated caps (100 VDC or above) be used for safety reasons?
Thank you very much!
 
quasimono said:
Quick question at @WKSmith regarding the components used to update the preamp:
Would Wima "MKS 2" series caps be sufficient for the voltages inside the preamp? Those are rated at 63 VDC. Or should higher rated caps (100 VDC or above) be used for safety reasons?
Thank you very much!
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WIMA 63v is an excellent choice for upgrading the FR-12! and it is well above the required rating for the 36v (ish) power supply rails

MKS is metalized polyester (Mylar) and a nice choice

MKP is metalized polypropylene and is a better choice for audio.

Their film/foil offerings are very nice for the small values.. especially in eq and crossover stages.

Use 5% tolerance if you can. And don't buy fake WIMAS on Amazon or eBay . There are several cheap fakes on there. (Which are probably still better than the stock Panasonic styled ones ... if we are being honest)

When I ordered 10's of thousands of capacitors in various values from a factory in Taiwan.. they were all spec'd based on the WIMA MKS line... but with only a 1% tolerance. Those are the grey ones you see in my builds
 
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