2. amps-preamps
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Does anyone care to ask an amplifier designer a technical question? My door is open.

I closed the cable and fuse thread because the trolls were making a mess of things. I hope they dont find me here.

I design Tube and Solid State power amps and preamps for Music Reference. I have a degree in Electrical Engineering, have trained my ears keenly to hear frequency response differences, distortion and pretty good at guessing SPL. Ive spent 40 years doing that as a tech, store owner, and designer.
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Perhaps someone would like to ask a question about how one designs a successfull amplifier? What determines damping factor and what damping factor does besides damping the woofer. There is an entirely different, I feel better way to look at damping and call it Regulation , which is 1/damping.

I like to tell true stories of my experience with others in this industry.

I have started a school which you can visit at http://berkeleyhifischool.com/ There you can see some of my presentations.

On YouTube go to the Music Reference channel to see how to design and build your own tube linestage. The series has over 200,000 views. You have to hit the video tab to see all.

I am not here to advertise for MR. Soon I will be making and posting more videos on YouTube. I don’t make any money off the videos, I just want to share knowledge and I hope others will share knowledge. Asking a good question is actually a display of your knowledge because you know enough to formulate a decent question.

Starting in January I plan to make these videos and post them on the HiFi school site and hosted on a new YouTube channel belonging to the school.


128x128ramtubes
atmasphere's avatar
atmasphere
11,861 posts
 
What Al said- The issue here is something called 'characteristic impedance'. The BNCs are designed for cables with a characteristic impedance of either 50 or 75 ohms. Without getting into the designer's head, it uncertain what CI the RCA connection is actually working with, since RCAs have no termination standard and are used with all sorts of impedances. Hence Al's comment about waveform quality; reflections of the signal (noise) may be the result. 

could you explain what shorting plugs are?
@bwguy
They are connectors that have all their connections shorted together. Installed at the input of an amp or preamp, in this way one can be sure that no noise is present at the input of that piece. 


ramtubes's avatar
ramtubes OP
460 posts
 

@ieales   Depends on the circuit design. A lot of pro gear has IC balanced outputs that can short either leg to Gnd and the output acts single ended, driving no current into the shorted leg
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iesles Please elaborate. I don't how the shorted lead drives no current?

AND if using balance lines, only connect the screen at the driven end.
A shield grounded only at the receiver forms pair of low-pass filters for common-mode noise.


I would think the screen should be conneted at both ends and the negative leg to the RCA body.
ramtubes's avatar
ramtubes OP
460 posts
 
@ffzz4 A couple tech questions for Roger. Others please chime in too.

Is it possible to estimate how much amp power will be enough (i.e. never clipping) simply based on speaker specs (assuming they are correct), and listening habit/situation (i.e. distance from speakers, loudness)?

If so, how?


Its rather easy. Imagine you listen at your speaker's real 1 watt sensitivity (2.83 volts for 8 ) ohms. If the speakers are relatively close and you sit at one meter and the sensitivity is 90dB and you listen at 93 then you are hearing 1 watt. The 3 dB extra is because you have two speakers. Lets say the room absorbs 3 dB and let your listening level be 90.

As  you move away for every doubling of distance you loose 6 db from a point source, 3 dB from a line source and O dB from a large planar. Anyone please correct me on that. I am doing it from memory.

If you want to listen at 100 dB rather than 90 you need 10 dB more power which is 10 watts. 110 dB 10 times more at 100 watts. 120 db takes 1000 watts. It gets out of hand in a hurry.

If you want to throw out your numbers i will work it out step by step. Most people with 100 watts only need 20 or less. But we all like $100 better than $20 and its rare to see a 90 watt amplifier isnt it.

David Manly always rated his 80 watt amps at 100 watts for just that reason.
ffzz
26 posts
 
My speakers are 93db sensitive (1w/1m) with 8 ohm impedance (7ohm min).

With a listening distance of 8 feet, it seems 15w will be more than enough to produce 100db or so loudness that Symphonic music peaks into from time to time.

Assuming 100db is my max need, will there be any advantage of using, say, a 30w amp rather than a 15w amp? What if the amp is class AB rather than A?






ieales's avatar
ieales
1,201 posts
 
Please elaborate. I don't how the shorted lead drives no current?

A cross-coupled opamp circuit wherein shorting one of the leads to 0V removes its drive. It was very common in the 70's & 80's. Later Analog Devices created monolithic driver with similar functionality and much better performance due to tighter on chip resistors and thermal tracking than discrete parts. Sorry I don't have examples or numbers.

I would think the screen should be connected at both ends and the negative leg to the RCA body.
I was speaking of Balanced like XLR. Single end Dual wires inside a screen should have the screen connected to the body at the driving end only and the -ve lead connected at both ends.
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