Although the Burr-Brown OPA637 and Lundahl LL1570XL transformers are an excellent combination to drive all normal loads I designed an output stage with enhanced dynamic performance even at heavy loads. This output stage should comprise the following specifications:
- high gain
- high output current
- wide bandwidth
- low distortion
High gain is advantageous to avoid redundant amplifying stages, high currents enable the output stage to drive heavy loads. But if both gain and currents are high there will be thermal feedback between output transistors and the front-end differential pair within the op amp causing distortion. This is an inherent limitation of single chip op amps.
Furthermore increasing closed-loop gain decreases the error reducing loop gain. Additionally, starting at relatively low frequencies, the loop gain rolls-off at 20dB/decade of signal frequency increase. In combination these effects can produce significant errors, especially at higher frequencies where the loop gain is low.
Current-feedback op amps, such as the OPA603, have good dynamic performance at both low and high gains. Unfortunately, DC performance of these op amps is poor. A composite amplifier using an OPA627 (see datasheet: http://www.kynix.com/uploadfiles/pdf65976/OPA637AM.pdf) and an OPA603 (see datasheet: http://www.klaus-boening.de/html/opamps.html#opa603) can combine the qualities of both. According to the Burr-Brown specifications the composite amplifier can drive 150Ohm loads to ±10V with no thermal feedback to the OPA627.
Fig.1: schematic of a balanced composite audio output stage. The gain of the OPA603 is set to 37 while the feedback loop of the OPA627 controls the overall gain.
Please note that the capacitors and resistors at the input may have different values in the phonostages because they are part of the RIAA network.
Figure 1 shows the schematic of the composite amplifier. The gain of the OPA603 is set to 37. The overall gain is set to 10000 / 20 = 500 (54db) or 10000 / 10 = 1000 (60db). Thus the OPA627 has a gain of 13.5 or 27 respectively. Since the OPA603 is in the feedback loop of the OPA627, the composite amplifier retains the excellent DC characteristics of the OPA627 at these relatively low gains. The typical offset measured is close to 50mV.
Contrary to the original Burr-Brown data sheets I used OPA627 instead of the recommended OPA637 (see Burr Brown application bulletin), because I was unable to control oscillation of an OPA637/603 compound, while the OPA627/603 compound runs smooth and stable in my application.
For optimal distortion behavior both the + and the - inputs of FET op amps should see the same impedance values. The 1.5k and 2.0k resistors minimize adverse effects of the existing imbalance of source impedances. While the - inputs see constant source impedances of 20Ohm, the + inputs see the RIAA network impedance varying between 1.5kOhm and 17Ohm across the audio band. Thus the impedance ratio is between 60 and 0.7 (see plot of impedance ratios). With the resistors added, the impedance ratio is between 1.5 and 0.75 from 20Hz and 20kHz.
The balanced design doubles the maximum voltage swing compared to a single ended op amp design and ensures an absolutely constant load of the power supply, independent from heavy bass notes or transients.
The Lundahl LL7902’s seem to be the right choice as suitable output transformers. They combine very high level capacity (+28dbU at 50Hz) with low copper resistance (see data sheet). Primary and secondary windings are separated by electrostatic shields. Depending on the winding connections, impedances can be transformed 1:1, 4:1 or 16:1. Figure 2a and 2b show the prototype of the output stage.
Fig.2a: Prototype of output stage with LL7902
Fig2b: Short feedback loop of the OPA603
With idle currents of 20mA of the bridged OPA603 and step down transformation as shown in the schematic, the output stage will run in pure class A mode at almost all loads and signal levels.
The output impedance is 20Ohm between 10Hz and 20kHz. Above 20kHz the impedance increases to 80Ohm at 100kHz (see plot of output impedances).
Figure 3 shows the frequency response of the output stage. The -6db point of the OPA627/603 compound at 54 db gain is beyond 2MHz. After the 4:1 step down transformation with the Lundahl LL7902 the frequency response is still flat up to 200kHz. The -6db point is around 1MHz. Hence the paradigm of a bandwidth one decade beyond the audio range is fully ensured.
Fig.3: Gain versus frequency plot of the composite op amp without and with output transformer.
The 39Ohm resistors at the outputs provide load isolation. The open loop impedance of the OPA603 is not zero. Thus complex loads may cause phase lag or delay of a signal from the output node and hence the feed back signal also suffers the delay. This may cause instability. A remedy is a series resistor added between the output node and the load to isolate the effect of load capacitance. The value was derived by trial and error for best square wave response in combination with the output transformer and the 220Ohm / 30nF combination (see Figure 4a and 4b).
Fig.4a: Output square wave response at 10kHz (3Vpp, gain 54db).
Fig.4b: Output square wave response at 100kHz (3Vpp, gain 54db).
In the meanwhile a MC head amplifier and RIAA network has been added to the enhanced output stage to complete the MK III phono stage “La Bohème ” (click here for the MK III page).
As time goes by.... the BB OPA 603 used in the high performance output stage is out of production.
In search for a suitable substitute listening tests were done with three alternative operational amplifiers, that matched the following criteria: 8 pin DIP package, supply voltage +/-15V, current feedback architecture.
Methods and Material
Selected opamps were the LM6181 (National Semiconductors), the LT1206 (Linear Technologies) and the AD811 (Analog Devices). Reference was the OPA603.
One channel of the MK III preamp received the opamps to be tested, the other was left with the OPA603. A-B-A listening tests were done after 24h breaking in time through one channel only to ensure identical conditions (same LP channel, same coil, same power amplifier channel, same speaker). The tests were done blinded. An experienced female person (former singer and musician) did the critical listening, while the author switched between the preamp channels. Music was chosen from acoustical sources exclusively (Jazz at the Pawnshop, Propius and Cantate Domino, Propius).
The LT1206 is not pin compatible to the OPA603. Hence it was soldered on a socket and rewired to simulate pin compatibility.
Within the listening impressions the LM6181 sounded a little harsher and overdetailed compared to the reference especially when listening to a soprano or chorus. The LT1206 exhibited a similar character, but was somehow cleaner and closer to the OPA603. The LT1206 may be a first choice opamp for those who preferentially listen to jazz and may be pop or rock.
The most promising substitute is the AD811 although the sonic character is a little smoother than that of the OPA603.
For best results the 1k feedback resistor should be reduced to 680 Ohm when using the AD811.