Hi Folks & Gurus!
so i recently acquired a brand new power stereo amplifier(dc coupled) , and a few things left me slightly puzzled. hoping some gurus here able to enlighten me.
I'm paring the power amp with my tube preamp(has capacitor at the output) , i measured using multi meter to confirm no dc output at the RCA terminals.
I also measured to ensure no DC offset at the amp speaker terminal , and result was less than 1mvdc.
However found that when playing slightly louder especially low frequency/ power demanding track would send the amplifier in to protected mode,according to manufacturer the amplifier has probably detect dc at the speaker terminal (threshold at around ~0.7vdc)? i did use my cheap multimeter to measure when playing those tracks , but the reading did not get anywhere near 0.7vdc, having said im seeing range from 30mv to 60mv before it trips , perhaps my multimeter not sensitive enough to pickup any spikes?
is it normal for a dc coupled amplifier to output any DC voltages at the speaker terminal when playing loud or playing low frequency notes?
Any response would be very much appreciated .
sorry that this is not a DIY related question , hopefully it is acceptable here 🙂
ps * amp is kinki studio ex-m7
so i recently acquired a brand new power stereo amplifier(dc coupled) , and a few things left me slightly puzzled. hoping some gurus here able to enlighten me.
I'm paring the power amp with my tube preamp(has capacitor at the output) , i measured using multi meter to confirm no dc output at the RCA terminals.
I also measured to ensure no DC offset at the amp speaker terminal , and result was less than 1mvdc.
However found that when playing slightly louder especially low frequency/ power demanding track would send the amplifier in to protected mode,according to manufacturer the amplifier has probably detect dc at the speaker terminal (threshold at around ~0.7vdc)? i did use my cheap multimeter to measure when playing those tracks , but the reading did not get anywhere near 0.7vdc, having said im seeing range from 30mv to 60mv before it trips , perhaps my multimeter not sensitive enough to pickup any spikes?
is it normal for a dc coupled amplifier to output any DC voltages at the speaker terminal when playing loud or playing low frequency notes?
Any response would be very much appreciated .
sorry that this is not a DIY related question , hopefully it is acceptable here 🙂
ps * amp is kinki studio ex-m7
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thanks for the reply, i tried without any speaker attached , the amp did the same thing, going in to protection so i dont think it is overloading it.
I guess that the amplifier input stage has no capacitor or perhaps a very large one and large signal transients are being detected as DC and that causes a problem. Simply place a 1uF film capacitor in series with the input + terminal. You can do this with very short leads, clips, tack soldering or whatever suits and retest without opening the amp. The idea is to isolate where the problem lies before diving in and messing with the internals.
thank you Ian!
yes there is no capacitor anywhere in the power amp , however my preamp has output capacitor? are you suggesting some of the DC leaked from the preamp?
the manufacturer is going to send me a new control board to increase the threshold to 1.3volt dc (from what i read it is still a safe threshold), just a little concerned that it might cause speaker damage later down the track.
yes there is no capacitor anywhere in the power amp , however my preamp has output capacitor? are you suggesting some of the DC leaked from the preamp?
the manufacturer is going to send me a new control board to increase the threshold to 1.3volt dc (from what i read it is still a safe threshold), just a little concerned that it might cause speaker damage later down the track.
i tried completely bypassing my tube preamp(direct from DAC to power amp) , it was still tripping the protection when playing some difficult tracks , maybe like Ian said? could it be false detection?
I'm uncertain what type of DC detection is employed and I suspect few people here yet know if or how a spike, surge etc. triggered it. The amplifier is a commercial product with no schematic or service info. but it seems to have an incredible amount of low frequency drive power or perhaps that turns out to be a form of pre-emphasis?
Apparently though, it hasn't been thoroughly tested with all types of preamplifier generally available, otherwise you wouldn't have this issue.
As indicated, mine is a quick test to ascertain how much of problem you have and the likely location. Without tests on unknown products, its difficult to have a constructive discussion about them, particularly unusual ones; their faults and resolution. As DIYs, I think we can expect that folk who seek help here are also prepared to obtain or maybe already have the gear to do any basic testing and simple repairs.
Apparently though, it hasn't been thoroughly tested with all types of preamplifier generally available, otherwise you wouldn't have this issue.
As indicated, mine is a quick test to ascertain how much of problem you have and the likely location. Without tests on unknown products, its difficult to have a constructive discussion about them, particularly unusual ones; their faults and resolution. As DIYs, I think we can expect that folk who seek help here are also prepared to obtain or maybe already have the gear to do any basic testing and simple repairs.
Hi Ian ,
Thanks for responding!
agree on your sentiment , my question is more towards understanding if fluctuating DC voltage at the amplifier output when playing music loud is normal or abnormal .
will probe the manufacturer more but doubt they will admit to any issues with their design.
found this site offers some info that my be relevant to my case :
Loudspeaker Protection and Muting
It is important to identify the lowest frequency likely to be passed to a speaker, because this determines the delay that must be introduced to prevent low frequencies from triggering the protection circuit (nuisance tripping). For practical purposes, a low frequency limit of 20Hz is satisfactory for a full range system, and this means that a minimum 25ms delay is essential. In reality, due to the combination of low frequencies, and asymmetrical waveforms at higher frequencies, a greater delay will normally be required. Unfortunately, the greater the delay, the greater the risk of drivers being damaged. In a full range system (i.e. using passive crossovers), midrange and tweeters will be offered some protection by the capacitors used in the crossover network, but these are missing in a biamped or triamped system. For this reason, it is important that the circuit can be easily modified to change the initial time delay before the system detects the DC and disconnects the speakers.
Loudspeaker Protection and Muting
It is important to identify the lowest frequency likely to be passed to a speaker, because this determines the delay that must be introduced to prevent low frequencies from triggering the protection circuit (nuisance tripping). For practical purposes, a low frequency limit of 20Hz is satisfactory for a full range system, and this means that a minimum 25ms delay is essential. In reality, due to the combination of low frequencies, and asymmetrical waveforms at higher frequencies, a greater delay will normally be required. Unfortunately, the greater the delay, the greater the risk of drivers being damaged. In a full range system (i.e. using passive crossovers), midrange and tweeters will be offered some protection by the capacitors used in the crossover network, but these are missing in a biamped or triamped system. For this reason, it is important that the circuit can be easily modified to change the initial time delay before the system detects the DC and disconnects the speakers.
Regarding fluctuating DC, a valid audio signal is actually fluctuating DC too and a super low amplifier frequency response means that more DC character of the signal will be passed within the protection time delay window.
Your quote is certainly fair comment by rode (Rod Elliott), as we would expect. However, he is talking about conventional types of DC protection in a conventional design amplifier which are intended to disconnect the speakers in the event of an amplifier fault as quickly as possible. Yours is a problem with the preamplifier output which may amount to the same but arise from a different cause and be a different matter to resolve.
The question is; what criteria the manufacturer used to arrive at their product's design and how was it tested for the specification of the DC detection? If it's Chinese product, I doubt you'll find any design details are available. Talking about the device without said details isn't going to help you much either. For the present, I think this a matter for the seller to resolve under warranty conditions, as it seems they have begun to.
Your quote is certainly fair comment by rode (Rod Elliott), as we would expect. However, he is talking about conventional types of DC protection in a conventional design amplifier which are intended to disconnect the speakers in the event of an amplifier fault as quickly as possible. Yours is a problem with the preamplifier output which may amount to the same but arise from a different cause and be a different matter to resolve.
The question is; what criteria the manufacturer used to arrive at their product's design and how was it tested for the specification of the DC detection? If it's Chinese product, I doubt you'll find any design details are available. Talking about the device without said details isn't going to help you much either. For the present, I think this a matter for the seller to resolve under warranty conditions, as it seems they have begun to.
The Amp is faulty and not the Speaker Protection..
Check this, read and try to understand what is possible from 2 400VAC Transformers and what not..
I doubt that this AMP is worth it's money.. to many Question Marks for me unbelievable SPECS. Especially the stated output power at 8Ohm.
There are many things which are not 'trustworthy'
Check out the Link and you will see it as well as I do..
Kinki Studio EX-M7 - power amp (black) Black. | Stereo \ Power tips Brands \ Kinki Studio Manufacturers \ Kinki Studio | Site map: Avcorp - Hi-Fi, Hi-end, audio/video cables and accessories
Preamp Matching
Please consider our award winning EX-P7 to match with the stereo power amp EX-M7. Our amplifiers are Direct-DC-Coupled without any capacitors in the signal path. This yield less colouration, high resolution, fast, and transparent sound quality.
Should you wish to use other preamp make, please be informed that we do not recommend the use of tube-based preamp.
You may experience slight pops/clicks noise to the loudspeakers, it's likely caused by the DC-offset in the preamp.
My statement to this:
Any Amplifier in this price class should be able to handle a Tube Preamp or line drive..
Check out the specs as well - to good to be true.. but one never knows..
Technical specification:
Parameters:
- Frequency Response: 0-2.5MHz (±3dB)
- THDN: ＜0.07% (-80db)
- S/N Ratio: >103db(0.01Hz- 1 MHz), >130db (A-Weighted)
- Output Power: 250 W RMS(2-8 Ohms)
- Max Instantaneous Power: 400w RMS (8 Ohms) How Could it possible be? RMS is continues POWER and not INSTANTANEOUS.! And for sure not MAX power. that is rather Peak to peak then..
- Max Output Voltage: 70VAC, 18A = where come this 18 Amps from
- Slew Rate: 200 V /us (Rise Time < 300ns)
- AC Power: 240VAC, 60Hz (Factory configure).
- Power Consumption: 30W Idle, 700W Max Output
- Input Sensitivity: 1.45Vrms
- Input Impedance: 51kΩ (Acceptable Range 10k-52kΩ)
- Input Connector: RCA x 1, XLR x 1
- Output: Speaker Binding Post 4mm L/R Channel Dimension and weight:
- Dimension: 375 W x 110 H x 430D
- Weight: 25 kg
Does the preamplifier also have no transition capacitors? Any displacement of the DC is amplified by PowerAmp and triggers the protection.
Possibly infra-low frequencies.
Possibly infra-low frequencies.
Gentlemen, forget about leaky caps and tube preamps: the OP has written clearly that the protection engages even when the preamp is bypassed.
The only reasonable question at this stage is at what level and frequency of the signal does the protection circuit engage. Is it possible the amp is being simply driven into clipping?
No idea what measurement equipment the OP has, but a simple voltmeter should suffice to verify this.
Using a software signal generator and a voltmeter at the speaker output with no speakers connected is should be easy to establish the level the protection kicks in. At various frequencies like 1kHz, 50Hz, 20Hz, 10Hz. If the voltage is above 40v when the protection engages it would seem everything works as expected. If lower, then the protection circuit is adjusted wrongly.
Checking the DC at output is also a good idea; it may be the offset is excessive to begin with and the addition of a low frequency signal makes it sufficient to trip the protection.
The only reasonable question at this stage is at what level and frequency of the signal does the protection circuit engage. Is it possible the amp is being simply driven into clipping?
No idea what measurement equipment the OP has, but a simple voltmeter should suffice to verify this.
Using a software signal generator and a voltmeter at the speaker output with no speakers connected is should be easy to establish the level the protection kicks in. At various frequencies like 1kHz, 50Hz, 20Hz, 10Hz. If the voltage is above 40v when the protection engages it would seem everything works as expected. If lower, then the protection circuit is adjusted wrongly.
Checking the DC at output is also a good idea; it may be the offset is excessive to begin with and the addition of a low frequency signal makes it sufficient to trip the protection.
HF self oscilations may trip DC protection. Improperly implemented DC servo may cause low frequency boost.
thanks all for the responses!
Correct ,without the tube preamp , it still triggers the protection.
Manufacturer is sending me an improved control board ( increased the trigger point to 1.3vdc), will report back if that fixes the issue or not.
will try testing the amp with generated frequencies too , as im interested to know if low frequency is indeed the culprit.
with the amp idle not paying anything there is no DC offset at all, so dont think it is due to high DC offset to begin with.
@analog_sa
I tried playing ultra low frequency track(starting 3 hz),without speaker attached , I could see right away on the multi meter it went as high as 800mvdc .. then protection kicked in .
Seems that plying low frequency is really the culprit ...
I tried playing ultra low frequency track(starting 3 hz),without speaker attached , I could see right away on the multi meter it went as high as 800mvdc .. then protection kicked in .
Seems that plying low frequency is really the culprit ...
Hi Chip_mk
Nope I dont think it has DC servo , just protection against high DC voltage , current threshold is around 0.7vdc , new board going to raise that to 1.3vdc
Nope I dont think it has DC servo , just protection against high DC voltage , current threshold is around 0.7vdc , new board going to raise that to 1.3vdc
3Hz is unreasonable. No signals below 10Hz would have any significant amplitude in recorded material and it is not surprising the protection circuit reacts.