# possible to remove high/low pass filters on an exsisting amp



## CraigRobbo

Hey all. I am building a subwoofer at the moment, and maybe able to get myself an old Mordaunt short plate amp, I was wondering is there any way to find/locate the high pass or low pass filters on the amplifier so that they can output all frequencies, that way I can run an external EQ


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## Da Wiz

If it is a 1-circuit-board system with all the adjustments and amp circuit on one board, I would think it would be **** difficult to know #1 if there were high pass or low pass filters built-in without original specs/info on the amplifier and even if you had that, you'd need a circuit diagram and knowledge of how to read the circuit diagram to find the high-pass/lo-pass filters. However, some home theater subwoofers do not have integral high-pass or low-pass filters because they knew that processors and AVRs could/would have high-pass and low-pass setting options and you do NOT want a passive and active crossover applied to an audio signal... in otherwords, if the subwoofer limits lows or highs, the AVR or processor settings for low-pass and high pass should be OFF. You could just drive the subwoofer with a half-decent single channel amp and only use settings in the AVR or processor to deal with frequencies above 150 Hz and below 20 Hz.


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## Da Wiz

Da Wiz said:


> If it is a 1-circuit-board system with all the adjustments and amp circuit on one board, I would think it would be **** difficult to know #1 if there were high pass or low pass filters built-in without original specs/info on the amplifier and even if you had that, you'd need a circuit diagram and knowledge of how to read the circuit diagram to find the high-pass/lo-pass filters. However, some home theater subwoofers do not have integral high-pass or low-pass filters because they knew that processors and AVRs could/would have high-pass and low-pass setting options and you do NOT want a passive and active crossover applied to an audio signal... in otherwords, if the subwoofer limits lows or highs, the AVR or processor settings for low-pass and high pass should be OFF. You could just drive the subwoofer with a half-decent single channel amp and only use settings in the AVR or processor to deal with frequencies above 150 Hz and below 20 Hz.


Replying to myself... a subwoofer without high-pass and low-pass filters built-in is a better subwoofer IMO. That allows you to use high-pass (and/or low-pass) filters in your surround processor. You should first determine if your subwoofer HAS LP/HP filters--it may not. Most subwoofers have so much mechanical roll off of high frequencies, they don't really need a HP filter. And most subwoofers don't need a LP filter unless you are playing warped LP record albums. You want all the low frequencies going to the subwoofer, no need in most subs to limit bass response with an LP filter.


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## hitokiri149

Hello, I am Device and Systems Engineer, and I will highly recommend NOT removing filtering in an amplifier. It is a vital part of how the system functions. A High-pass filter is used to block DC voltages going into the transistor and helps with level shifting so the AC component of the signal is within the nonlinear component of the transistor. The Low-pass filters not only remove frequencies that are too high for the transistor to respond to, eg above 1MHz. It also provided much needed capacitance for driving the inductive load that is the speaker driver. That is just on the fundamentals. Engineers would have also computed other factors particular to that amp. Without these filter voltage/current spikes would destroy the transistor.


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## hitokiri149

Da Wiz said:


> Replying to myself... a subwoofer without high-pass and low-pass filters built-in is a better subwoofer IMO. That allows you to use high-pass (and/or low-pass) filters in your surround processor. You should first determine if your subwoofer HAS LP/HP filters--it may not. Most subwoofers have so much mechanical roll off of high frequencies, they don't really need a HP filter. And most subwoofers don't need a LP filter unless you are playing warped LP record albums. You want all the low frequencies going to the subwoofer, no need in most subs to limit bass response with an LP filter.


A Low-pass filter can be as simple as just a capacitor between the signal and ground . All amplifiers will have this.


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## Da Wiz

hitokiri149 said:


> A Low-pass filter can be as simple as just a capacitor between the signal and ground . All amplifiers will have this.


All amplifiers for home theater subwoofers do not have low-pass filters or high pass filters as being discussed in terms of a home theater system. The surround sound processor has low pass/high pass active digital domain filters. It is VERY bad to have an active low pass/high pass filter in the audio processor and a passive low-pass or high-pass filter in the subwoofer. There should be ONE passive or active low-pass/high-pass filter in the signal path and no more than one. Two filters in series is very bad for phase and signal integrity. We are NOT talking about a DC-blocking capacitor in the output of an amplifier (aka AC-coupled). That is a safety device present in most, but not all audio amplifiers. There are some audiophile amplifiers that do not have DC blocking capacitors in the output of the amplifier (DC coupled) because those capacitors change the sound a little bit depending on materials used and construction of the capacitor.


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## hitokiri149

Da Wiz said:


> All amplifiers for home theater subwoofers do not have low-pass filters or high pass filters as being discussed in terms of a home theater system. The surround sound processor has low pass/high pass active digital domain filters. It is VERY bad to have an active low pass/high pass filter in the audio processor and a passive low-pass or high-pass filter in the subwoofer. There should be ONE passive or active low-pass/high-pass filter in the signal path and no more than one. Two filters in series is very bad for phase and signal integrity. We are NOT talking about a DC-blocking capacitor in the output of an amplifier (aka AC-coupled). That is a safety device present in most, but not all audio amplifiers. There are some audiophile amplifiers that do not have DC blocking capacitors in the output of the amplifier (DC coupled) because those capacitors change the sound a little bit depending on materials used and construction of the capacitor.


Sorry, I misunderstood. What caught my attention was what seemed to be making adjustments to internals of the amplifier, and I was focusing on the high power amplifier, since *CraigRobbo *was asking about a subwoofer. When, you said, "you'd need a circuit diagram," I was trying to warn against adjusting filtering components, since they can also be a functional part of the amplifier circuit. You can't just by-pass them without thinking about the whole system. If the system was built with the option to disable this filtering then no problem. If not, you could by-pass them and maybe the system runs fine for decades and sounds great; or maybe you get 60 seconds out of it, all the while is sounds like a dying moose. I still stand by my recommendation of not removing the filtering, if the amplifier is not built to have it disabled.

With that, I do support learning about these types of things; I find https://www.homemade-circuits.com/class-d-amplifier-circuit-using-ic-555/ to be a set of fun circuits you could build in a home workshop on perfboard without setting you back a mint.


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