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Old 6th October 2015, 04:37 PM   #1
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Default OP amp phono preamp gain

Just wanted to double check that I am calculating this gain right.

There are two gain stages

so the first one has

(3320/150)+1 or 23.1333 gain and the second gain stage has the same

so multiplying these tother

23.13*23.13=534.9969


and then converting this to dB using this calculator

Decibels to Voltage Gain and Loss convert calculation conversion amplification amplifier electronics - sengpielaudio Sengpiel Berlin

gives me a gain of 54.5dB

am I doing this correct. The reason I ask is that I just tried an off the shelf phono that is supposed to have 40dB of gain but seems to have much more than my DIY phono that I am calculating has 55dB
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Old 6th October 2015, 04:53 PM   #2
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realized i forgot to attach the image of the phono preamp design.

Click the image to open in full size.
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Old 6th October 2015, 05:46 PM   #3
rayma is offline rayma  United States
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Quote:
Originally Posted by stevovee View Post
gives me a gain of 54.5dB am I doing this correct.
Yes, for low frequency gain. Then in the midband you lose 20dB due to the RIAA network,
so the nominal signal gain is 34.5dB. So, the other preamp has 5.5dB more gain.
This one will likely have much better RIAA response and lower noise, though.

Last edited by rayma; 6th October 2015 at 06:07 PM.
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Old 6th October 2015, 05:56 PM   #4
AndrewT is offline AndrewT  Scotland
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If you need the extra gain, then put it all into the first stage. Try 75r instead of 150r.

But don't put in too much, or you'll use up all the gain bandwidth product.
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Old 7th October 2015, 12:40 PM   #5
jcx is offline jcx  United States
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put at least 1 pole of the RIAA roll off in the 1st amp feedback and you don't have a GBW problem
and get dynamic headroom benefit too for mistracking, pops, ticks

does cost a extra RC low pass to correct the Zero introduced by the non-inverting low pass filter's transfer function flattening out at unity gain


the flat gain blocks with passive RC EQ in the middle is the most demanding of op amp performance compared to intelligent low pass frequency shaping in the feedback network(s)

it really seems wrong that those suspicious of feedback amp performance tend to select this worst case topology

Last edited by jcx; 7th October 2015 at 12:45 PM.
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