Shure discret preamp with EQ.
Shure SR107 Discrete EQ
- Thread starter iomegaman
- Start date
Help Support GroupDIY Audio Forum:
That output transformer is strange. The block diagram shows that the “line output” tap is -2dB (so something close to a 1:1 or 1.25:1 turns ratio), but it says the “mic output” is -52dB!
-52dB is a huge ratio…something like 400:1. If we assume that the primary impedance is around 10K (it really shouldn’t be any higher if it has a something near a 1:1 tap for line out), the output impedance of the mic winding would be something like 0.0625ohms. That can’t be right, can it?
-52dB is a huge ratio…something like 400:1. If we assume that the primary impedance is around 10K (it really shouldn’t be any higher if it has a something near a 1:1 tap for line out), the output impedance of the mic winding would be something like 0.0625ohms. That can’t be right, can it?
I just picked up one I'll measure it.
Ok just measure the Transformer here's the actual data:
Primary (blue/red on schematic) = 66.4 Ohms
Secondary (white/green on schematic) = 66.4 Ohms
Secondary (brown/yellow) = .4 Ohms
For the "Mic line" output feature (brown/yellow):
Using .078 to determine turns a=ratio that makes it a 12.884 :1 step down for the mic...
If you read the manual it has a setting to run it out kind of like an inline EQ for consoles that use a "Link" jack it outputs a really low mic level there.
Primary (blue/red on schematic) = 66.4 Ohms
Secondary (white/green on schematic) = 66.4 Ohms
Secondary (brown/yellow) = .4 Ohms
For the "Mic line" output feature (brown/yellow):
Using .078 to determine turns a=ratio that makes it a 12.884 :1 step down for the mic...
If you read the manual it has a setting to run it out kind of like an inline EQ for consoles that use a "Link" jack it outputs a really low mic level there.
Huh. 12.884:1 is only around -22dB, not -52dB. I wonder why they marked it that way.
I have no idea the input transformer is:
Primary= 27 Ohms
Secondary = @ 600 ohms...I double checked its wired that way...
Green/yellow to XLR= 27 Ohms....black and white to amplifier board = 597 Ohms...1:4.7 step up...
Possible error in the schematic data?
Walrus
Well-known member
From schematic, level pre transformer is 0.21 Volts. Mic output Post transformer says 0.6 mV.
That calculates as -50.88 dB.
That calculates as -50.88 dB.
That's what I'm questioning. A 0.21V to 0.6mV drop would require a transformer with a turns ratio of 350:1
@iomegaman said he measured it at ~13:1
That's a big discrepancy.
Also, @abbey road d enfer (or other admin) - I shouldn't have commented on this in the first place (I didn't realize I was in the Technical Docs section), sorry. Is it possible to move this conversation elsewhere?
