Newbie question reduce volume Bryston 7BST following AVC-1?

discussion of magnetic volume control design for both line and speaker levels.

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JoeFagan
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Newbie question reduce volume Bryston 7BST following AVC-1?

Post by JoeFagan »

Have AVC-1 driving Bryston7BST with 0.033UF to roll off bass below 50Hz and AVC-1 second output to Marchand XM9 crossed over 100Hz then to W4Sound ST1000. Problem is 7BST is rated at 1.0V while ST1000 is rated at 2.95V. Result is having difficulty matching levels of the two amps even with the XM9 set to plus 6 (maximum gain ) . The 7BST drives Magnepan 3.7 and the ST1000 drives Magnepan Tynpani IIIa Bass panels ( 40 years old ) ( two per side ). Would like to be able to attenuate the level of the 7BST such that I could match the 3.7s and Tympani Bass Panels. Probably 2 to 4 db would be enough. I can solder well enough and follow simple instructions but rather ignorant about electronic theory. Thanks in advance even if it means that I can't fix this issue. Joe Fagan
dave slagle
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Post by dave slagle »

Hey,

What is an AVC-1?

It seems an additional 1-6dB attenuator before the amp that needs further attenuation would do the trick. This could be soldered to a 6 position switch and allow further gain matching.

dave
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JoeFagan
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Post by JoeFagan »

Thanks for the responding so quickly. I may have the name wrong. I am referring to the Autoformer Preamp/Kit sold by Bent Audio/John Chapman and I thought you as well. Regardless it is an Autoformer Preamp and I am using the RCA outputs. You are correct that a reduction in the output of the Bryston 7BST of 1 to 6 db would solve the problem. I was concerned that adding any kind of second attenuator between the "AVC-1" and the 7BST might be problematic particularly as I have added a capacitor to my IC. When I purchased the "AVC-1" preamp I believe that I talked to you about its compatibility with the Bryston 7BST amp. As I recall you were most gracious with your time and expertise. Joe Fagan
dave slagle
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Post by dave slagle »

The second attenuator will not cause a problem and would be the most economical way to go short of a resistive pad.

dave
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JoeFagan
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Post by JoeFagan »

Thanks for resolving that issue. I do have one more question. I have not been able to find the AVC-1 Output Impedance which is apparently necessary along with the Input Impedance of the 7BST (50K ) to calculate the values of the two resistors to attenuate the volume of the 7BST. Could you provide the Output Impedance of the AVC-1? By the way the AVC-1 is by a large margin the best "preamp" that I have ever had. Thanks in advance for your continued help. Joe Fagan
JoeFagan
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Post by JoeFagan »

Mr. Slagle I hate to continue to impose on you but my attempts to understand how to determine the output impedance of the AVC-1 have only confused me more. All that I know is that as the volume on the AVC-1 goes down that the output impedance goes down. I would really appreciate it if you could give me an approximate number for the AVC-1 output impedance that I could use to calculate a two resistor attenuator for a 7BST with input impedance of 50K. In my reading about the AVC-1 it seemed that the volume level of the AVC-1 most frequently used was important. In my case the volume range that I use with various sources ranges from about 30 to 40.Thanks in advance. Joe Fagan
dave slagle
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Post by dave slagle »

The output impedance depends on the output impedance of the source and the volume setting. worst case (aside from the gain settings 55-61)is when set to 54 which is unity. At that setting the output impedance (Z) of the AVC is the same as the output Z of the source. If you attenuate 6dB (set to 48) then you have cut the voltage ratio by half and since the output impedance is the voltage ratio squared you now have 1/4 the output impedance of the source. If you continue another 6dB (42) nets 1/16th the source impedance and it keeps going down.

Onto your filter question. Within reason (assuming Zsource is 10% or less than Zload) the source Z has no effect on the filter behavior so you can just use the 50K

dave
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top three traces are 1R 100R and 1KR   Bottom trace is 10K
top three traces are 1R 100R and 1KR Bottom trace is 10K
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JoeFagan
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Post by JoeFagan »

Thanks again. As I understand Z source is the 7BST ( 50K ). Thus my Phono, and AV Processor would be 4% and 2% respectively. Well within your 10% or less. My Sony SACD output impedance is unfortunately 10K which makes it 20% of Z source. Fortunately the Sony sounds great. Given you filter diagram as I understand it I would place a 50K resistor between the positive and ground wires in the IC and place a 1R,100R or 1000R in the positive lead after the capacitor. Is this correct? Are there advantages to any of the three resistor values? Particularly considering that the Sony is outside the parameter you recommended.Hope that I have understood the issues and am not wasting too much of your time. Joe Fagan
dave slagle
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Post by dave slagle »

The input of the 7B is the load. I also highly doubt the output impedance of the SACD is 10K. !0 K may be listed as the maximum load but I cannot imagine the output Z being anywhere near that high.

The simulation just shows haow changing the source Z changes the filter behavior. Simply put a 0.033uf in series with the input of the 7B and you are done. (assuming the 7B really has a 50K input Z.

dave
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JoeFagan
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Post by JoeFagan »

Dave thank you very much. I have just ordered two Z Foil Vishay 49.9K resistors from M. Percy and hope to be able to install them in a few days. If in the future I wanted to increase or decrease the attenuation would that be possible by increasing or decreasing the size of the resistor? Have you considered making an Autoformer Preamp similar to the AVC-1 that I have available again in kit form? I realize that you provide the essential parts but for people like me who have at best a minimal understanding of electronics it would be better if it was a partially assembled kit. Anyway thank you again for sharing your expertise with me. Joe Fagan
JoeFagan
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Post by JoeFagan »

Mr. Slagle: unfortunately the 50K resistors connected to the positive and negative IC wires seems to have no effect on the volume of the Bryston 7BST connected to the AVC-1 Autoformer preamp. Measurement by Stereophile CD with Radio Shack Sound Meter. The Stereophile CD was played on a Sony 5400ES SACD player. Any explanation and/or suggestions. Joe Fagan
dave slagle
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Post by dave slagle »

Hello Joe,

Simply adding a 50K resistor in parallel to the input impedance of the amp (assumed to be 50K) will not cause any attenuation. The 50K value in my simulations is the input impedance of the amps and was not meant to be an additional resistor. Based on the 0.033u cap adding another 50K inparallel will only shift the crossover point of the filter up one ocatave which may be audible but it will be really subtle and easy to miss if you are not looking ot hear it.

If you want to adjust the level you will need a potentiometer (volume control) and this is a three terminal device (three connections in, out and ground). The same result can be found with two resistors in series across the input with junction of the pair feeding the amp. Using a pair of 25K resistors will net you a 6dB cut in level. In order to keep the filter (crossover) behaving as desired you will need to make sure that R1 + (R2||Rload)=50K. Since the 50K value of the amp input isn't going to be changed than you are best putting the atteuation network before the 0.033u cap or you will need to change the cap value to fit your new input impedance.

dave
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Screen shot 2016-01-19 at 11.05.56 AM.png (12.64 KiB) Viewed 10944 times
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JoeFagan
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Post by JoeFagan »

Mr. Slagle I realize that you are probably too busy to provide individual consultation to someone such as my self who has limited theoretical knowledge regarding electronics. I thought that I had made myself clear that I wanted to know the output impedance of the AVC-1 but apparently there is not a stable output impedance. Secondly I wanted to calculate the size of the two resistors that I would need for each IC to attenuate my Bryston 7BST amp by up to 6db given that the 7BST input resistance was 50K. Third I wanted to know whether to put the capacitor to roll off the bass before or after the resistors. Unfortunately I must not have been clear as to my needs. Even with your last email the situation remains unclear to me. However, I will not bother you anymore as you are very busy. I will find someone else with sufficient time to address my issues. Joe Fagan
dave slagle
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Post by dave slagle »

a pair of 5K resistors should work fine and the 0.033uf cap should go at the output. This will give you 6dB of attenuation and provide a 10K load to the source. The output impedance of the attenuator will be 2.5K so it should be kept as close to the amp as possible.

dave
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