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Old 14th April 2017, 04:58 AM   #1
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Default RIAA Help

Hi,

I'm having some trouble with the RIAA on a project I've been assembling. Everything works as it should but the bass is down. It uses a passive RIAA network and while there are a lot of online calculators, I really need some help to know which RIAA components (R7, R8, C2 or C4) to change and whether to increase or decrease.

I've attached the schematic and my RIAA plot.

Cheers!
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File Type: jpg My Project.jpg (53.8 KB, 259 views)
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Old 14th April 2017, 04:16 PM   #2
DF96 is offline DF96  England
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Not an easy circuit to analyse, but it appears that LF boost depends on the input impedance of the second stage, which depends on the current gain of Q3. R9 is boostrapped so that will not contribute very much to input loading. As BJT current gain is a very poorly controlled characteristic then in this sense it is quite a poor circuit.

Try putting a higher gain transistor for Q3.
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Old 14th April 2017, 04:32 PM   #3
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The Q5 stage also has an ill defined gain for the same reasons.
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Old 14th April 2017, 04:57 PM   #4
DF96 is offline DF96  England
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Yes, I hadn't spotted that. Is R17 a stopper? It seems to have quite a high value.

This circuit, although dated 2002, looks more like something from 1972. Even with the right RIAA response it is likely to have significant distortion.
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Old 14th April 2017, 07:26 PM   #5
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I forgot to mention that I used BC549 transistors instead of BC109. I measured the hfe of all the BC549's and got about hfe=600 for Q3. I could try a higher gain transistor but would dropping R7 cause my bass response to increase?

Are their any high gain suggestions for the transistors?...I'm mostly a tube guy....

Yes, I would not be surprised if it is from the 1970's!
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Old 15th April 2017, 07:50 AM   #6
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Just tried dropping R7 by 20% which resulted in about 1% drop in dB across the band upto 1khz. I ran out of time to try anything else but will look at again in a couple of days.
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Old 15th April 2017, 08:44 AM   #7
AndrewT is offline AndrewT  Scotland
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I am calculating ~ +13V on the collectors of the input LTP.
Is that about right?
It looks like a similar voltage appears on the collectors of the second stage LTP.
Is that about right?
That would give about 2mA through the EF output stage.
The whole circuit runs at extremely low current.
Would it work better with some higher currents in each stage?
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Old 15th April 2017, 09:48 AM   #8
DF96 is offline DF96  England
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The bass boost is essentially set by the ratio of Q3 input impedance to R8, so you could try reducing R8 - but then you would need to change the RIAA capacitors too.

No point in spending too much time on a bad circuit. Find a better circuit.
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Old 16th April 2017, 09:13 AM   #9
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Any suggestions for alternate circuits that are worth co considering df96? ��
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Old 16th April 2017, 09:30 AM   #10
Mooly is offline Mooly  United Kingdom
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Why don't you try simulating your circuit and feed it from a (simulated) inverse RIAA network. You could then judge the basic performance and dependencies and also then move on to tweak it.
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