Hi all,
When I first became interested in DIY electronics, I really wanted to try to make a Plate Reverb. I naively thought it would be a fun little project to keep me busy for a couple of weekends, the concept behind how it works is quite simple but it wasn't long before I realised this was way above my skillset and had to put this idea on the shelf.
A year has passed, my interest in electronics has been sparked again and I'm feeling more confident to try and tackle this task again. I spent the weekend putting together a schematic for this project. I'm feeling pretty confident that it could work how it is intended but due to my lack of experience and the fact that certain parts of the schematic were done by guess work, I'm putting this out there in the hope that some kind soul might tell me where I'm going wrong. Fingers crossed I've not messed up my calculations.
To summarise the intention behind the circuit, I want to be able to drive 2x4ohm transducers (likely between 10-20W) from a line level input to excite a metal sheet thats going to be around 500x300mm (closer to an EMT 240 than 140). The power amp section should have a good amount of headroom but I'm not overly concerned with clipping and noise since I'm trying to excite a metal plate not build a HiFi system. I want to have control over low and high frequencies both pre and post reverb. I will be using piezo discs to pick up the sound from the plate that I assume will need a preamp to bring to line level and suitable output impedance. I have added in feedback network as a means of controlling the reverb time.
Key Points:
- The inputs and output of the circuit should be able to function as mono or stereo and will be balanced line level.
- The nominal input voltage will be +4dbu or 1.228 vrms, by my calculations, once the input signals have been unbalanced and summed the voltage will be 4.912v.
- At a supply voltage of +/- 15v, the LM1875 should be able to output roughly 10W, the circuit has been designed so the nominal input level will leave around 15db of headroom (not including the feedback network).
- The LM1875 has a minimum gain of 10. I will need to reduce the input voltage by about 30db to give me my desired headroom.
- The idea behind R8 and RV3 was to give me the required attenuation then to have control over the input level to the LM1875, I'm not sure if this will work but I think it will?
- The transducer impedance will be 8 ohms (or two 4 ohm transducers in series)
- I want a 2 pole Low and High pass filter with variable cutoff points on both the input and output signals, how I have gone about this is partly guesswork.
- The values from the TL072 and onwards have either not yet been chosen or are subject to change, the plan is to first measure the voltage that the piezo discs output then work out how much gain will be required
- I wasn't sure how I should approach the feedback network but what I have put in the schematic seemed like my best bet
Questions:
- Does the secondary impedance of the input transformers work as if they were in series with the resistors going to the input of the summing amplifier, changing the values needed for R3 and R13 or is what I have done here fine?
- The components around the LM1875 were mostly copied from the data sheet. I have not included a capacitor in the feedback network for the other op amps, should I be doing this to avoid DC offset?
- I believe the NE5532 needs a load impedance of no less than 2k. In this circuit is the load impedance the same as the output transformers primary in parallel to RV8/RV9 or just the primary itself?
- How should I calculate the values for R28/29 to give me unity gain, I think I’ve done it wrong here but should it be all of the resistors before the NE5532 in parallel? - on second thoughts if this is the case things would be quite unstable because of RV2, RV5 and RV7 acting as variable resistors. Maybe I should learn how to implement a Sallen-key filter instead?
- I now think the voltage divider in the feedback loop is unnecessary if the signals going into the summing amp are at line level, I was forgetting about the 6db gain I was getting from balancing the input signals.
- Maybe I should put a buffer after the pot in the feeadback network?
I appreciate this is a bit of a chunky post so no expectations for anyone to read it all the way through but would be very grateful to anyone that can chime in on this.
Cheers,
Sam
When I first became interested in DIY electronics, I really wanted to try to make a Plate Reverb. I naively thought it would be a fun little project to keep me busy for a couple of weekends, the concept behind how it works is quite simple but it wasn't long before I realised this was way above my skillset and had to put this idea on the shelf.
A year has passed, my interest in electronics has been sparked again and I'm feeling more confident to try and tackle this task again. I spent the weekend putting together a schematic for this project. I'm feeling pretty confident that it could work how it is intended but due to my lack of experience and the fact that certain parts of the schematic were done by guess work, I'm putting this out there in the hope that some kind soul might tell me where I'm going wrong. Fingers crossed I've not messed up my calculations.
To summarise the intention behind the circuit, I want to be able to drive 2x4ohm transducers (likely between 10-20W) from a line level input to excite a metal sheet thats going to be around 500x300mm (closer to an EMT 240 than 140). The power amp section should have a good amount of headroom but I'm not overly concerned with clipping and noise since I'm trying to excite a metal plate not build a HiFi system. I want to have control over low and high frequencies both pre and post reverb. I will be using piezo discs to pick up the sound from the plate that I assume will need a preamp to bring to line level and suitable output impedance. I have added in feedback network as a means of controlling the reverb time.
Key Points:
- The inputs and output of the circuit should be able to function as mono or stereo and will be balanced line level.
- The nominal input voltage will be +4dbu or 1.228 vrms, by my calculations, once the input signals have been unbalanced and summed the voltage will be 4.912v.
- At a supply voltage of +/- 15v, the LM1875 should be able to output roughly 10W, the circuit has been designed so the nominal input level will leave around 15db of headroom (not including the feedback network).
- The LM1875 has a minimum gain of 10. I will need to reduce the input voltage by about 30db to give me my desired headroom.
- The idea behind R8 and RV3 was to give me the required attenuation then to have control over the input level to the LM1875, I'm not sure if this will work but I think it will?
- The transducer impedance will be 8 ohms (or two 4 ohm transducers in series)
- I want a 2 pole Low and High pass filter with variable cutoff points on both the input and output signals, how I have gone about this is partly guesswork.
- The values from the TL072 and onwards have either not yet been chosen or are subject to change, the plan is to first measure the voltage that the piezo discs output then work out how much gain will be required
- I wasn't sure how I should approach the feedback network but what I have put in the schematic seemed like my best bet
Questions:
- Does the secondary impedance of the input transformers work as if they were in series with the resistors going to the input of the summing amplifier, changing the values needed for R3 and R13 or is what I have done here fine?
- The components around the LM1875 were mostly copied from the data sheet. I have not included a capacitor in the feedback network for the other op amps, should I be doing this to avoid DC offset?
- I believe the NE5532 needs a load impedance of no less than 2k. In this circuit is the load impedance the same as the output transformers primary in parallel to RV8/RV9 or just the primary itself?
- How should I calculate the values for R28/29 to give me unity gain, I think I’ve done it wrong here but should it be all of the resistors before the NE5532 in parallel? - on second thoughts if this is the case things would be quite unstable because of RV2, RV5 and RV7 acting as variable resistors. Maybe I should learn how to implement a Sallen-key filter instead?
- I now think the voltage divider in the feedback loop is unnecessary if the signals going into the summing amp are at line level, I was forgetting about the 6db gain I was getting from balancing the input signals.
- Maybe I should put a buffer after the pot in the feeadback network?
I appreciate this is a bit of a chunky post so no expectations for anyone to read it all the way through but would be very grateful to anyone that can chime in on this.
Cheers,
Sam
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