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Simple opamp preamp issues
Simple opamp preamp issues
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Old 14th September 2019, 12:18 AM   #1
Sadface is offline Sadface  New Zealand
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Default Simple opamp preamp issues

Hi Guys,

I recently build a Jaycar KC5159 preamp.
Universal Stereo Preamplifier Kit | Jaycar Electronics New Zealand

kc 5159 tape circuit.jpg

It comes with different equalisation options: RIAA, Tape, and mic.
I chose tape as that sounded like the closest to a DAC source (Musical Fidelity Xdac V3).

I changed 1 or 2 values to use some nicer parts I had lying around. I changed the 47uf bipolar caps on the input to .47uf polyester caps.
I also changed the 100R resistor on the output to 150R as I had a some nice dale 150R resistors.

I also wired a generic jaycar 50k log pot on the input for volume control.
kc 5159 tape circuit used.jpg

Everything tested fine, the circuit functioned but as some might guess had some sound issues.

1) The volume was extremely sensitive with the finest adjustment taking volume from quiet to deafening. My current thinking is that this is due to the 100k resistor in parallel with the log pot at the input.

2) Music just sounded weird. My thinking is that this is due to the tape equalisation applied in the feedback network which from the curves I've seen is designed to boost base response and roll off higher frequencies.

This is what I am looking at for version 2.
1) Removal of the equalisation by substituting the feedback network resistors for 10k on both sides
2) Decreased gain as I don't need a huge amount. A gain of 2 should be sufficient I think.

3) Removing the inductor on the input as it doesn't seem necessary from a digital source.
4)Changing the volume control to a 100k linear pot with a 12k parallel resistor as per Rod Elliot's a better volume control ESP - A Better Volume Control

Here is what I have come up with.

kc 5159 rework.jpg

Is this likely to have the desired outcome?
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Old 14th September 2019, 12:22 AM   #2
rayma is offline rayma  United States
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Much better, just add a 100k to ground after the input coupling capacitor.
The gain of two may be enough for your system.
Also remember the decoupling capacitors on the rails.

Last edited by rayma; 14th September 2019 at 12:25 AM.
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Old 14th September 2019, 01:20 AM   #3
JMFahey is offline JMFahey  Argentina
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Even better if you can use LOG/Audio volume pots.
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Design/make/service musical stuff in Buenos Aires, Argentina, since 1969.
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Old 14th September 2019, 03:29 AM   #4
rayma is offline rayma  United States
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The output capacitor is small, unless used with a tube amplifier.
If you want to experiment further, sub pp types for the polyester caps.

Last edited by rayma; 14th September 2019 at 03:33 AM.
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Old 14th September 2019, 05:08 AM   #5
Sadface is offline Sadface  New Zealand
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Quote:
Originally Posted by rayma View Post
Much better, just add a 100k to ground after the input coupling capacitor.

like so?
kc 5159 rework.jpg


The board has pads for 100nf decoupling caps on each supply rail.
I'm using some Wima mks2 caps I had lying around as I didn't like the look of the no name stuff that came with the kit.


Regarding the coupling caps; I am going to order some wima mks2 1uf 63vdc caps for input and output since I want to replace the LM833N that came with the kit.
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Old 14th September 2019, 01:57 PM   #6
Ken Newton is offline Ken Newton  United States
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Quote:
Originally Posted by Sadface View Post
The following three potential issues are apparent:

1) You can/should remove the 100k resistor at the input (just after the 1uF input coupling cap) and relocate the 100k volume pot. there. Which, I believe, is what rayma has suggested. Locating the volume pot. after the input coupling cap would ensure that no D.C. appears across the volume pot. That could be an issue because, as it is now, changing the volume would also change the level of any D.C. applied to the input coupling cap. Yes, the input cap. would block that D.C., but there would be a low frequency transient produced each time you change the volume setting, typically resulting in a 'swooshing' sound. No physical harm is probably done to your system, but it could be an audible annoyance, besides making your preamp scream amateur DIY.

2) you can bypass (eliminate) the 47uF bipolar capacitor connecting the feedback network to ground. In other words, terminate the network directly to ground. Yes, the closed loop gain at D.C. will double from unity, however, that shouldn't present a problem as any input D.C. is already blocked by the 1uF input coupling cap. Therefore, the doubling in D.C. gain only affects the op-amp's intrinsic voltage offset. Which is specified at 700uV worst case, which therefore becomes 1.4mV worst case. Either of which is completely blocked by the 1uF output coupling capacitor. Which leads to point three.

3) The 1uF output capacitor is probably too small, unless, as was indicated by rayma, the power amplifier has a very high input impedance. Meaning, roughly 100k or higher. The impedance defining the required value of the output coupling capacitor is the 1M resistor connecting the output cap. to ground in parallel with the input impedance of the next stage. An undersized output cap. will cause bass roll-off. A rough rule-of-thumb I like to use for relating the required size of the output coupling capacitor to the input impedance of the next stage it is coupled to is:

100k > 1uF
50k > 2uF
25k > 4uF
20k > 5uF
10k > 10uF

Last edited by Ken Newton; 14th September 2019 at 02:21 PM.
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Old 14th September 2019, 02:00 PM   #7
Mark Tillotson is offline Mark Tillotson
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Quote:
Originally Posted by Sadface View Post
2) Music just sounded weird. My thinking is that this is due to the tape equalisation applied in the feedback network which from the curves I've seen is designed to boost base response and roll off higher frequencies.
A line preamp shouldn't be doing any equalization by default. It might have tone controls to adjust for room response or personal preference, but equalization is a gross distortion of frequency response used to undo the opposite gross distortion of frequency response inherent in a vinyl or tape playback cartridge/head.

This is a preamp for vinyl, tape head, or dynamic microphone only.
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Old 14th September 2019, 09:56 PM   #8
Ken Newton is offline Ken Newton  United States
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I overlooked that you are utilizing the linear pot. based volume control of Elliott Sound Lab. That 12k resistor on the wiper will greatly lower the resistance as seen across the pot., depending on the wiper's setting. This would likely necessitate a larger value for the input coupling cap., so as to not cause bass roll-off. This is how circuits typically present themselves, you usually have to give up something in order to gain something else.
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Old 15th September 2019, 01:04 AM   #9
Sadface is offline Sadface  New Zealand
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Regarding the output cap value.If I understand correctly:

A) The output coupling cap and the 1M parallel resistor form a high pass filter.

B) Assuming premise (A) is correct. I would also think that this filter also includes the input impedance of the load in parallel with the 1M on the output. In this case that is 25k input impedance for a Marchand Electronics XM9 active crossover.

C)Assuming that (B) is correct that would give an ESR of:
ESR=product over the sum
=(1000000*25000)/(1000000+25000)

=24390.24

giving a corner frequency of:
Fo= 1/(2*Pi*RC)
=1(2*pi*24930.24*0.000001)
=6.53Hz

Assuming that (c) is correct, if I was to leave the existing .33uf cap in there it would give a corner frequency of 19.78hz which seems like an acceptable response (tube power amps).

Is my working correct or have I missed something?

Last edited by Sadface; 15th September 2019 at 01:11 AM.
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Old 15th September 2019, 01:32 AM   #10
rayma is offline rayma  United States
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Seems ok. A good tube amplifier should be able to handle that easily.
Some design for a bandwidth of as low as 2Hz.
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