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Old 9th February 2018, 04:21 PM   #1041
rsavas is offline rsavas  Canada
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Up to you how you want to do it, adding a 600 ohm source R is not the end of the world, actually helps in some circumstances.

I do not think that 9ppm 1KHz/1V is a problem, too caught up in a numbers game. I am pretty sure a NE5532/4 does not achieve this figure and it is acceptable to many like Doug Self. I would bet that whatever signal (commercial music) that you are listening to has already gone through multiple NE5532/4 or equivalents before it gets to your ears. What's another one or two added by the consumer gear = non issue.
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Old 15th February 2018, 05:24 PM   #1042
nauta is offline nauta  Canada
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Hi Guys

Blu Glo: If you read my first paragraph (post-1038) you see I refer to power-up and power-down. Some symmetric rails do notrise and fall equally, and some circuits do not evfen perform properly until close to the designed rail voltage is achieved, so the muting is placed at the OUTPUT of the preamp so that none of the thumps or oscillation gets to the next unit.

The relay contact is in parallel with the signal path so its quality is of little concern.

Jfets or BJTs can also be used as shunt switches for AC signals. You have to make sure that their nonlinear capacitance when off does not impair THD. Yes, a BJT can switch AC signals as long as the peak value is less than 6V, as the transistors zeners at around 7V.

If muting the output is aesthetically displeasing, with the concern being that signal present will cause the circuit driving the short to over-heat etc, then add a second mute point earlier in the chain. The early mute will presumably be at a benign signal point and keeps the subsequent stage from trying to force a signal into a short. The output mute still protects downstream equipment from receiving thumps etc.

There are very good jfets with low Rds-on of just a few ohms. Not exotic except as through-hole parts - haha. As a shunt switch, a series resistance improves the effective attenuation. To improve things further, you can parallel tw or more jfets. Take this a step further an cascde two or more R-jfet sections with single jfets per section.
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Old 16th February 2018, 04:28 AM   #1043
blu_glo is offline blu_glo  England
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Hi Nauta, yes I meant on the preamp output; but the actuator shorting the following equipments' input (ie pre out socket only) to ground rather than the preamp op-amp's output itself. In that way also no transients are passed on to following equipment. I have also used signal relays in series with the signal to no detrimental effect whatsoever.

Last edited by blu_glo; 16th February 2018 at 04:34 AM.
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Old 17th February 2018, 01:09 AM   #1044
nauta is offline nauta  Canada
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Hi Guys

blu-glo: I never said anything about the preamp output opamp. Most preamps have a capcitively-coupled output and/or a low-value series resistance. The relay shunt would be after these and is electrically-equivalnet to having it right at the input of the next unit, but with the benefit that the preamp won't thump into any amp connected to it.

If your system thumps, yu can always make an outboard speaker turn-on delay using relays. However, this puts the contact in series with the speaker and can introduce THD. You can get gold-contacts rated for fairly high current and that would be the way to go. Absolutely stay away from silver plated contacts in relays and switches if the contacts handle audio directly. For a short to ground they are okay but not for series signals.

Also, there is no reason for the series-R to be 600R. The usual value is 10-68R.
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Old 17th February 2018, 09:23 AM   #1045
blu_glo is offline blu_glo  England
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I am familiar with relay setups as I have done this for years, I was just concerned about possibility of opamp having its output not-quite-shorted (600R, 10-68R, I happen to use 47R almost universally) if the gear is switched to mute mode with signal still present..... not an ideal situation but in most cases I suppose no harm is done.
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Old 20th March 2018, 05:30 PM   #1046
dennismiller55 is offline dennismiller55  United States
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Well I have had some time to really enjoy this preamplifier since my last post #1003.

I wish to discuss the Log Law Level LED (LLLL) circuit. I am trying to understand how to interprete and effectively use the Log Law Level LED (LLLL) to set the correct gain of the phono stage. A few members have also asked this question in the various forum post discussing this preamplifier.

This is my understanding of how the LLLL works and how I set the gain of my phono stage.

According to the magazine article and block diagram, the phono gain should be adjusted to optain a 1V (1kHz) at the output to effectively drive the line amplifier stage. The correct gain is achieved when the LLLL approximately blinks on and off equally.

Over the last few weeks I played several different recordings of mixed genres. I have adjusted the total gain of the MM phono stage to 45dB to get approximately an equal on and off blinking of the indicator LED.

So how do I know this is right? Is my gain of 45dB correct for my cartridge? Is the LLLL working correctly?

I think it is based on this.

I am using an AT-155LC MM cartridge. According to the specifications sheet for this cartridge it has a 5mV output level (1kHz, 5cm/sec). I found an online Blog article titled “Taking the Guesswork out of Phonostage Gain”.

To summerize the article I used the formula of 20 Log V1/V2 = NdB

V1= output voltage (desired 1V). V2 = 5mV (cartridge output level). NdB = Ideal phono stage gain

20 log (1V/.005V) = 46dB

The 46dB validates the gain of 45dB that I chose and the LLLL circuit is working as designed.

I hope my assumptions are correct and gives some useful insight to the LLLL circuit.
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