Topic: Main Amp & Pre- Amp Interconnection

[amos1660]

My esteemed collegues in the community, I would like to seek your professional advice on the ideal connection between my Adcom Pre-amp GFP-565 and Adcom Main Amp GFA 555. Presently I'm using the bypass output of the pre-amp and connect it to the input of of the main amp for stereo listening. However; I'd like to add a subwoofer to test its effect on the system. May I be guided by your expertise as to where I should connect the left and right leads from the powered sub to the pre- amp. Besides the bypass output, there are 2 main outputs of the pre-amp namely Lab and Normal.

Your valued inputs/feedbacks would be much appreciated. Thank you and warm regards.

[av_phile1]

I would use the pre-amp's bypass output to connect to the subwoofer's L and R input and use the Normal output to your main amp.  That's mainly because the sub won't be able to use the pre-amp's tone controls anyway while you can still use them in the Normal connection.  You may be a purist who don't want tone controls but in a 2.1 set-up, you will need to reduce the bass on the main speakers via the bass tone control and let the subwoofer deliver more of the deep bass.  Otherwise, you would get overlapping bass frequencies from both and will peak to sound boomy.  The bypass connection to the subwoofer won't be affected by your bass control setting so the sub gets all the bass signals.  

I'm assuming your main speakers are bass-challenged below 50hz and the subwoofer can go down to 20hz more effortlessly.  There's such a thing as cutover or crossover point between main speakers and subwoofer so the transition across the audio spectrum can be seamlessly shared between them.  You are essentially taking the bi-amping configuration.  Your pre-amp's bass control is often specified at a certain frequency where the control attenuates to the max typically something like -12 db at 120hz. Thie bass controls can serve as your electronic crossover in the bi-amping set-up.  Refer to your Adcom manual.   Check if your sub can be set at 120hz crossover point then. Many subs specify their -6 or -12db crossover points (the upper filter, not the lower one).  That should approximate your crossover point.  Then turn the pre-amp's bass control to your main speakers all the way down and start listening, adjust according to your listening taste.  You will have to play it by ear to get a smooth transition between the main speaker and sub and minimize peaking frequencies resulting from having both deliver the same set of bass frequencies.   An SPL meter and a test CD with frequency sweep below 100Hz can help you there.  Hope this helps.

Does your subwoofer have a high pass L and R line out?  I ask become some subs do for connection to another amp.   If it does, then connect your subwoofer to the pre-amp's bypass out, then connect the sub's filtered L and R line out to your main amp.  That way, you get a better crossover between sub and main since your main speakers will be delivering a high-pass frequency set filtered out from your sub, trapping the low bass frequencies for the sub to work on.

[amos1660]

I would use the pre-amp's bypass output to connect to the subwoofer's L and R input and use the Normal output to your main amp.  That's mainly because the sub won't be able to use the pre-amp's tone controls anyway while you can still use them in the Normal connection.  You may be a purist who don't want tone controls but in a 2.1 set-up, you will need to reduce the bass on the main speakers via the bass tone control and let the subwoofer deliver more of the deep bass.  Otherwise, you would get overlapping bass frequencies from both and will peak to sound boomy.  The bypass connection to the subwoofer won't be affected by your bass control setting so the sub gets all the bass signals.  

I'm assuming your main speakers are bass-challenged below 50hz and the subwoofer can go down to 20hz more effortlessly.  There's such a thing as cutover or crossover point between main speakers and subwoofer so the transition across the audio spectrum can be seamlessly shared between them.  You are essentially taking the bi-amping configuration.  Your pre-amp's bass control is often specified at a certain frequency where the control attenuates to the max typically something like -12 db at 120hz. Thie bass controls can serve as your electronic crossover in the bi-amping set-up.  Refer to your Adcom manual.   Check if your sub can be set at 120hz crossover point then. Many subs specify their -6 or -12db crossover points (the upper filter, not the lower one).  That should approximate your crossover point.  Then turn the pre-amp's bass control to your main speakers all the way down and start listening, adjust according to your listening taste.  You will have to play it by ear to get a smooth transition between the main speaker and sub and minimize peaking frequencies resulting from having both deliver the same set of bass frequencies.   An SPL meter and a test CD with frequency sweep below 100Hz can help you there.  Hope this helps.

Does your subwoofer have a high pass L and R line out?  I ask become some subs do for connection to another amp.   If it does, then connect your subwoofer to the pre-amp's bypass out, then connect the sub's filtered L and R line out to your main amp.  That way, you get a better crossover between sub and main since your main speakers will be delivering a high-pass frequency set filtered out from your sub, trapping the low bass frequencies for the sub to work on.


My dear friend av_phile 1, thanks a lot for a very enlightening and comprehensive reply to my query. I'll do what you have recommended. By the way I'm using a pair of Magnepan speakers for stereo listening and will utilize a JBL 12 sub. Again many thanks, you're very kind.

[mar]

av_phile.....I have the same set up gfp 565, gfa 555II ,cayin cd17 and bw 802...with bw aws 700..........
                connected as you suggested........sounds good..........if I upgrade my
                pre amp with tube type...........do I get that warm sonic improvement associated with
                vaccuum tube system.....your advice will be very much appreciated.........

[av_phile1]

Yes.  A neutral power amp simply  amplifies whatever signal you feed to it including the "warm" sound of your tube preamp. 

[mar]

Yes.  A neutral power amp simply  amplifies whatever signal you feed to it including the "warm" sound of your tube preamp. 

  many thanks to the insights. Now my next question is ( if you dont mind ) my cd player
             have a balance output....do i get sonic advantage if i use a tube pre amp with this
             type of input....... considering that i have to connect the pre amp out via rca input
             of my power amp

[av_phile1]

The advantage of using balanced XLR connection can be had if the balanced signal is processed all the way.  That means, your CD player actually has 2 DACs with separate L and R balanced circuits outputting a balanced analog signal and not just a single DAC with unbalanced output converted into a balanced signal using a phase shifting Op Amp, your preamp has a balanced circuit topology to process a balanced signal without converting it to an unbalance signal anywhere in the signal path and your main power amp also processes balanced signal.  

With your set-up the only benefit from using XLR is better electrical interference management at the input stage between CD player and preamp. Preamp output signals are stronger with lower impedances which are less susceptible to noixe pick up compared with input signals.  

In the first place, balanced XLR connecton belongs in the professional audio recording and playback environment where the use of long cables to carry weak signals with high impedances are subject to sonic degradation from electro-magnetic interferences.  These are commonly signals from microphones and instrument pick-ups where a noise cancelling XLR connection is recomended.  

Having said that, it's not necessary for home playback equipment where cables are short.  If you have it in your CD player and preamp, you can use it for peace of mind. Check if your CD player has twin DACs.   Not all CD players have and unless you use kilometric cables between your components, RCA cables are just fine.  You can always try them out and see if you can hear any difference.  There's supposed to be a 3db loudness advantage when using XLR if implemented properly.  For me, I prefer a digital SPDIF connection between a CD player and a preamp if only to ensure uncolored neutral signals that otherwise get colored by op-amps in different CD players' analog output stages.  But since your preamp is valve, you won't have this.  

[mar]

av_phile I  ...........That was a very informative reply..............many thanks
                          its good to know we have people like you  in our  industry........
                   

[av_phile1]

Thank you.  Just sharing what I know.